I just won a chess game because in the endgame my opponent was in zugzwang.

Zugzwang (German for "compulsion to move") is a situation found in chess and other games wherein one player is put at a disadvantage because they must make a move when they would prefer to pass and not move. The fact that the player is compelled to move means that their position will become significantly weaker. A player is said to be "in zugzwang" when any possible move will worsen their position.

Positions with zugzwang occur fairly often in chess endgames. According to John Nunn, positions of reciprocal zugzwang are surprisingly important in the analysis of endgames.

The best place to play chess or watch chess games is at https://lichess.org. Everything is free including computer analysis of games.