A trap in chess is a sequence of moves designed to lure the opponent into a specific mistake, typically by presenting an opportunity that appears attractive but is actually a tactical illusion.
Traps work on a psychological level: the opponent sees what they want to see. A piece may look freely capturable, but taking it triggers a devastating response — a fork, a checkmate, or an even greater material loss. The effectiveness of a trap depends heavily on surprise and on the fact that the punishing reply is not immediately obvious to the opponent.
In practice, traps are especially common in the opening phase. There are well-known sequences where an unprepared player can lose a piece — or even the game — within the first ten moves. Learning the typical traps in your opening repertoire allows you both to set them and to avoid falling into them.