The brush is the person responsible for maintaining the sign-up lists for poker at poker rooms.
Poker, unlike most forms of gambling, requires a certain "critical mass" of people in a game in order for the game to be enjoyable for most players. Usually, this number is around 6, and most poker rooms try hard to maintain games at a level of 8 or more players per table (this is different for Seven-card stud games, which can only seat a maximum of 8 players due to the number of cards used).
Keeping the active tables "full" is a tricky job, and requires a person to be in charge of deciding where new players are placed when they enter a room (to keep tables more balanced), when to open a new table to handle waiting players, which player is allowed to join a table next, which tables to "break" (shut down) in order to disperse their players to other short-handed tables, and in extreme cases, which players to move from one table to another in order to maintain balance.
This position of authority is called the brush. The brush is the person you will need to speak to first upon entering a poker room, and will often decide which table you are seated at. You want to be nice to the brush; not only do they have a difficult and demanding job, but they can steer you towards a better game if they like you, or keep you in mind for a table change if you request it.
A brush often works in conjunction with a floorman to manage the workings of the poker room, though in small card rooms, the brush and floorman may be the same person.
