Opposition is the most fundamental king-versus-king concept in endgames. When two kings stand directly opposed with one square between them, the player whose turn it is to move is at a disadvantage because they must retreat or step sideways, allowing the opponent's king to advance and gain ground.
The most common application of opposition is in king and pawn endgames, where having the opposition often determines whether a pawn can be promoted or whether the defending side can hold a draw. For example, with a single pawn, the attacking king must achieve the opposition to shepherd the pawn to the promotion square.
Beyond direct opposition (kings separated by one square), there is also distant opposition (kings separated by 3 or 5 squares on the same line) and diagonal opposition. Understanding these concepts and the related idea of corresponding squares is essential for mastering pawn endgames, which are the foundation of all endgame technique.