We find it hard to improve on the wikipedia definition:
A live action role-playing game (LARP) is a form of role-playing game where the participants physically portray their characters. 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 refs decide the setting and rules to be used and facilitate play.https://en.wikipedia.org/wiki/Live_action_role-playing_game
Crucible, specifically, is a game in which the players pretend to be vampires, living on a South London council estate, where they scheme against one another and prey on the inhabitants. Our fiction takes inspiration from books like Sunglasses After Dark and Night Junkies or films like Near Dark and Let The Right One In (as well as more traditional vampire works like Dracula or Interview with the Vampire).
There’s a joke about Vampire LARP, that summarises the game as “Let’s play dress-up and act petty”. This is both not wholly inaccurate, and a lot more fun than it sounds.