Tempo is a term that originated from Italy. It means time (the chess time we talked about in the previous article). We will go more into detail on this notion in this article.
You may have run into this notion by now and, if you haven't, it's alright, you'll learn all you need to know about it in this article. When you'll find this term you will may find it in expressions like : "the move was made with tempo" or "the player lost a tempo" or something similar to that.
You should know that time in chess (tempo) can be won or lost . How is that, you may ask? Very simple! I said earlier, that in the beginning of the game both players try to develop their pieces as better and as faster as possible. If a player is forced to make a retreat of his piece than that means that he lost a tempo. Basically, if he moves a piece to a square and then he moves it back is like he never performed that particular move (so, he lost a move, a tempo; is like his opponent made two moves while he had made only one)
In the next game White attacks the black queen with 1. Bg5. But Black has a good response: 1... f6. Now the bishop must be taken to a safe location 2. Be3. You can see that the bishop could have been moved at e3 from the beginning. Now White had lost a tempo (it's like Black had made two moves while White made only one)
As a side note: although you should always avoid making this kind of moves if you happen to be in such a position don't be afraid of taking back your move (don't do something rushed like capturing the pawn from f6 in the example above). Usually it's better to lose tempo than to lose a valuable piece.