Table and rules
Side pot in poker
Definition
A separate pot formed when a player is all-in and the other players keep betting among themselves.
A player who is all-in can only win the part of the pot they have matched, which is the main pot. Bets above that go into a side pot that only the players who are still active can win.
With several all-in players of different stack sizes there can be several side pots, each contested by fewer players.
Example
A has 100, B has 300, C has 300. All three go all-in or call: the main pot of 300 is for all three, the side pot of 400 is for B and C only.