In chess, a tempo refers to a single unit of time — one move. Gaining a tempo means making one more useful move than your opponent, while losing a tempo means wasting a move without any tangible benefit.
The tempo advantage becomes critical in sharp tactical positions or tight endgames. For instance, if you attack an opponent’s piece while simultaneously developing one of your own, you gain a tempo: your opponent must react to your threat, giving you the freedom to continue your plan uninterrupted. Conversely, moving the same piece twice in the opening without good reason hands your opponent a free tempo — and with it, the initiative.
In practice, before each move, ask yourself: does this move force a response, or does it let my opponent do whatever they want? A move that creates a concrete threat (a capture, a check, an attack on an undefended piece) makes good use of your tempo. A purely waiting move that threatens nothing may be a free gift to the other side of the board.