A "fork" in chess refers to a tactic where a single piece attacks two or more of the opponent's pieces at the same time, creating a situation where the opponent cannot defend all of the threatened pieces. This often leads to gaining material, as the opponent can only move one piece to safety.
Forks can be made by different pieces, but they are most commonly associated with knights due to their unique movement, which allows them to attack multiple pieces simultaneously. For example, a knight can land on a square that attacks both an opponent's queen and rook, forcing the opponent to choose which piece to save.
A fork can also occur with other pieces, like queens, bishops, or pawns, but the concept is the same: a piece threatens multiple valuable targets in one move.