In a trébuchet both kings attack the enemy pawn while defending their own; whoever is to move must abandon his pawn. Arrive so that the opponent is on move.
The trébuchet (a medieval siege engine that tips over) is the simplest mutual zugzwang in pawn endings. White's king on c5 attacks d5 and defends d4; if Black's king stood on e4 it would attack d4 and defend d5. In that picture whoever is to move loses his pawn, because every king move drops the defence.