A poker buy in is the amount of money you must put up to enter a real money game. In a cash game, this is often between 40 and 200 big blinds. In a tournament, this is a fixed amount to play, with the top finishers receiving a scaled percentage of the total prizepool based on where they finish, less a fee taken by the house (rake).
A poker tournament may be listed as a $20+2 tournament, meaning $20 goes into the prizepool and $2 is rake going to the house (or casino) for hosting the game. A tournament like this with 1,000 players would have 1000*20=$20,000 in prizes to distribute among the top finishers, often between 14% and 20% of the field receiving some prize. Those who finish closer to 1st place receive a larger percentage of the $20,000 prizepool.