What is LARP?
A live action role-playing game (LARP) is a form of role-playing game where the participants physically act out their characters' actions. The players pursue goals within a fictional setting represented by the real world, while interacting with each other in character. The outcome of player actions may be mediated by game rules, or determined by consensus among players. Event arrangers called gamemasters decide the setting and rules to be used and facilitate play.
Quoted from the Wikipedia article about LARP.
So, I completely become my character?
Yes. You dress up like your character and act like your character. From a certain point in time the GMs are calling "in-time", which means that from now on every action is taken in the game. This includes (but is of course not limited to) battles, magic spells and camp life.
How does it basically work?
The basics are very similar to tabletop. People meet up and start role playing. This does include camp life which gets acted out. Due to it's nature of characters running physically around (sometimes even in big areas and big numbers), GMs can not be present at all times. This especially includes battles, and for certain actions (rituals f.e.) GMs need to be called to be present.
What about pauses or breaks?
If you need to take a break, take it in game. Normally the game gets only interrupted if something special is happening or in emergencies. The word "Stop/Hold" does have a special meaning, it means that something happened which is not part of the game in any way. If somebody gets injured during battle, "Stop/Hold" is yelled, everyone immediately has to cease fighting and also yell it to acknowledge the hold.
There's also the possibility that you get ordered by the GMs to close your eyes and ears during a time-freeze. This technique is used to surprise the players, f.e. if a magician is calling his skeleton army. The players get time frozen, and NPCs are lining up and the game is continued, for your character there's no delay and the skeletons appeared out of thin air.
If there are 300 people clashing against each other, who counts the HPs?
The players do, only for their own character of course. It's impossible to check the HPs of every single player in a massive battle, so you'll have to trust every single player.