RebuyIn contrast to a freezeout tournament, in which a player is irrevocably eliminated when all his chips are gone, re-buy tournaments allow eliminated or shortstacked players to purchase more chips. Usually, this procedure, the rebuy, is such that as soon as a player's stack is below the original starting amount for the tournament, he may pay the original buy-in and recieve the starting amount of chips again. This is only possible at the beginning of the tournament. After the rebuy phase, the play is normal freezeout.
In a $20 rebuy tournament in which every player gets 2000 chips, a player can buy an additional 2000 chips for $20 as soon as he has fewer than 2000 chips. Instead of leaving the tournament when he's lost all his chips, he can buy in a second, third, fourth...time and start all over.