Conventions
Any group of people engaged in a group activity will, over time, develop its own shorthand and quirks. Archers talk about form, draw weight, draw length, optics, arrow flight speed, and lots of bow-related things that don't really have any use outside of that context. Computer types can have entire conversations only tangentially related to English at all. Larps are no different.
In this case, we have a number of game conventions that everyone just sort of seems to know without ever really having been told. We pick it up from each other and pass it along by example to new arrivals. It's not ideal and it contributes some avoidable length to the learning curve. In the interest of easing that transition, these are some of the things that are common in the game and useless out of that context.
Common Conventions:
Contested Roll: Some skills test the player’s skill against a set objective, namely Disable Device, Resurrection, and Tracking. To determine the success or failure of such the following steps are taken. First the player adds up all of their bonuses, and then subtracts the negatives set by the Marshal, task, or the item being worked on. If the total modifier is positive the player will add it to their roll, if negative it plays no real effect unless the total is -10, in which case the character will automatically fail the roll. Conversely, if the final modifier is +9 or more the roll automatically succeeds. The character will roll a D10 and add their modifier, if positive; if the combined total is 10 or better then they succeed on the challenge.
Counted Actions: Counted actions exist, loosely, to handle two conditions. One, you want to do something that's perfectly viable for your character but out of the question for you. If your character has Wings, for example, they're free to leave the ground, but you probably can't sprout your own wings and fly. Two, you need to take some specific action for which rules provide a fixed delay - refitting your armor, for example, takes sixty seconds and it technically a sort of sixty count. Binding Strike is counted to let you know how long you have before you get eaten or whatever. Killing Blow takes a three count which sounds fast and it is if no one's trying to stop you. However you arrive there, all counted actions work the same way: You start at the end, announce what you're doing and start at the top of the count, then count down, repeating the announcement until you finish at "I whatever 1", at which point your action is completed and everyone should react accordingly. You may see this a lot on modules, where Marshals have to handle player actions we didn't expect - it's easier and smoother to assign a count than call a meeting of the Rules Team and argue about it.
Crowns: Crowns are points awarded for doings that the help the game - donating props and reps, NPC'ing, anything that's beneficial to the game as a whole is probably worth some crowns. They're accumulated and spent in a variety of ways. They can be turned in towards extra Skill Points, or used to purchase a variety of in-game items.
Healer vs. Medic: If someone calls for a Healer, they mean a Mage or Paladin with in-game healing effects to solve in-game trouble. If someone calls for a Medic, there's an out of game situation that needs qualified medical personnel to attend. Similarly, if your character has been jacked up by NPC's, you need a Healer, not a Medic. Medic is called rarely enough that this is easy to remember; it's mentioned here because of various services that train "Medic" as a crisis response.
Hold: This one is an actual rule, but not an actual rule that fits conveniently anywhere. "HOLD", usually yelled like it owes someone money, causes the game to cease immediately. Holds are typically called for safety issues (if you notice your opponent is about to back up into a table or a hole in the ground, call a hold and warn them), very often for glasses that have gone flying off of their designated face, sometimes for more serious medical issues. Because Hold is invoked for real-life situations, we take it very seriously. If someone calls a hold, repeat it, stop playing, and do not move. More than one pair of glasses has been lost to overly cavalier treatment of a hold. Hold is also used sometimes to clarify rules - combat can get loud and confusing and it's easy at first to lose track of what happened to you or what you're allowed to do about it. Once it's been established that a hold was called for rules, you can relax a little. Don't move, but feel free to pull some tags, or grab a drink from your water bottle, collect some close-by packets, etc. Until it's established that the hold is a rules issue and there is no out of game danger, hold still, don't talk, and wait it out. Holds are rarely lengthy. When whatever situation that caused the hold is addressed, someone will check to make sure everyone's ready and then call Lay On - a Lay On is the opposite of a hold, resuming game play at the point where the Hold left off.
Item Tags: These are the out-of-game indicators of in-game possessions. You can have a beautifully constructed boffer sword, but you can't actually use in the course of the game without a corresponding item tag for it. Lots of tags come attached to reps. Some are just tags - very few people bother with reps for their poison stashes beyond the packets they intend to throw at people.
Machine Gunning or Drum Rolling: These are two terms used interchangeably to mean the same thing - using a melee weapon like a drumstick to drill damage way faster than any real sword ever could be. Some people can get unbelievably fast on swings, especially with dual wield and identical damage. It's not legal, there's a Flurry rule for this exact reason, but people, especially newer people, get carried away and excited and before they know it, they're inflicting damage at a rate that burst-fire rifles envy. It's not usually done maliciously and pretty obvious when it is. If someone calls this to you, take a deep breath and slow your swings down. If you're under attack and it feels more like a big foam centipede is running on you than it does like someone is swinging a foam stick, point it out but remember it's probably not personal or intended to give offense.
Out-of-Game: A character that's out of game is neither affected by nor can affect the state of the game world. Ideally this is indicated by a white headband - some people (like Plot) spend more time out of game than in, running modules and overseeing stuff. If you're less commonly out of game, or just need a quick minute to retie your boots, putting your hand or your weapon on your head is accepted shorthand for "briefly out of game".
Reps: You'll see this a lot in the rules and you'll hear it a good bit on site. A rep is a physical representation of an in-game item. "Physical representation" being unwieldy and people being people, this was inevitably shortened to "physrep", which you'll still hear from time to time, and then "rep", which you'll hear a lot. Your boffer long sword is a rep for an in-game long sword. Your armor rep is whatever you wear around to justify using armor points when you get hit. It sounds pretty self-explanatory and for the most part it is, but there are some murky areas, too. Spell packets can, variously, be reps for poison vials, thrown weapons, or literally nothing (for mages and other spellslingers.) Reps have a necessary but not tightly-coupled association with...
Reset: Given ten minutes of peace, you can reset your character sheet once per event. This refreshes all of your refreshable skills - spell pool, mana, Energy, Ley Points and any dailies or per-reset skills. This typically also resets your Body to its starting value and clears negative effects, but some effects do stick around through a reset. You'll see this a lot in Powers and some Skills - many Powers function twice per reset, for example, so you have two uses when you first check in and two more after you Reset. Reset, like the name implies, resets your skills - if you had mana or energy or per-reset skills left over when you started your reset, they're lost when it completes.
Swimming/Going to the Rock: FE's resurrection mechanic is a Lifewell, or viewed differently, the world's worst pool in which you never want to swim. "Sending someone swimming" or "Sending someone to the pool" means sending someone off to resurrect. A previous game used a cruelly-imprisoned, enslaved sentient stone as its mechanic and some of the vets of that game still say "sending someone to the rock" to mean the same thing.
Tank: Drawn from MMO culture, a Tank is someone you can trust to stand between you and harm. There's a large overlap with folks in thick armor with shields and reams of their own defenses, but there are plenty of heavy-armor warriors that will hang you out to dry if there's a chance they can get a goblin by running off after it. Most tanks are Champions or Paladins. Not all Champions or Paladins are tanks. You may also hear this used as a verb - "tanking a monster" means trading hits with it, "tanking damage" just means absorbing it so someone else doesn't have to.
"What do I see": Very frequently, NPC's will be out and about dressed in a tabard and some weapons, but they could be playing any number of things. Since we don't have the budget or the time to fully dress out every NPC in perfectly-identifiable costuming, you'll hear "What do I see" called - this is an out of game question to clarify what creature a character is looking at in-game.
Underwater Combat: Combat underwater has several rules that differ from the norm. Each step underwater requires a three count, though this does not need to be vocalized. All weapons, Fire Traps, and Massive Mechanical Traps deal half damage underwater. Spells may only be touch cast and require both hands to be free. Acid, Bombs, Gas Traps, and Vials may not be used. Gun do not function under water. A character may hold their breath for two minutes if they are not in combat, but combat reduces this time to a single minute. If a character runs out of breath, they are unconscious and begin to drown, becoming dead after a minute. Having the skill for Underwater Combat will lessen these penalties.