The minimum amount of money required by a player to sit down in a particular poker game.
The buy in is the entry fee for any particular table or tournament.
Tournament buy-ins are usually displayed like the following Party Poker example:
Limit Hold Em $20+$2 - The total payable would be $22. $20 to the prize fund and $2 to Party Poker for arranging the event
Cash (ring) table buy-ins are often displayed like this Party Poker table example:
$200 NL - This means the maximum you can bring to the table in cash is $200, NL means the game is No Limit.