Each move takes one tempo. Thus a player can be said to "lose a tempo" when taking one more move than necessary to achieve something, or to "gain a tempo" when taking only two moves to do what would have taken three by other methods, for example.
In some endgame situations, a player must actually lose a tempo to make progress. When the two Kings stand in opposition, for example, the player to move is often at a disadvantage (zugzwang) and so must use the triangulation in order to return to the same position but with the opponent to move.
Source: "Wikipedia, The free Encyclopedia"