A split pot happens when two or more players have the exact same winning hand at a showdown. In this situation, the pot is equally divided among all players holding the winning hand.
For example, in Hold’em, a board runs out As Jh Jd Jc 2c. At the showdown, three players have an ace in their hand, for Jacks full of Aces. Nobody has the last Jack, so this is the winning hand, and all three of them will equally split the pot. If there is $300 in the middle, each player will get $100.
Another application of this term is in the context of split-pot games. These are variations like Omaha Hi-Lo and Stud Hi-Lo. In these poker variants, the pot is split between the best high and the best low hand at the showdown. If there is no qualifying low hand, the best high hand wins the entire pot.