Tempo is a fundamental concept that manifests in every phase of the game. In the opening, developing a piece with a threat (like attacking the opponent's queen) gains a tempo because the opponent must spend a move responding to the threat instead of pursuing their own development.
In the middlegame, tempo considerations drive tactical calculations. A zwischenzug (in-between move) gains a tempo by inserting a forcing move before completing an expected sequence. Check is the ultimate tempo-gainer because it absolutely forces a response, allowing the checking player to pursue other plans.
In endgames, tempo can determine the outcome. Concepts like triangulation and zugzwang revolve around tempo manipulation: in triangulation, a player spends an extra move to pass the obligation to move to the opponent, while in zugzwang, the side that must move would prefer to pass because any move worsens their position. Mastering tempo is essential for competitive chess at all levels.