When a player goes all-in, they commit all of their chips to the pot, either as a bet or a call. If other players still have more chips after matching the all-in bet, they can continue betting in a side pot, but the all-in player is only eligible to win the main pot. The hand proceeds to showdown if there are no further raises.
All-in Example
For example, if you have $100 in chips and push them all into the middle during a hand, you are going all-in. Other players can either call your bet or fold, and the hand will go to a showdown if called.