It's sort of a "free move". When you make a move that forces the opponent to have to react to it -- like if you make him move his queen -- you're said to be gaining a tempo.
what is tempo

each move is done with a purpose, either to develop a piece, attack, set a trap . . . . When you have to make a move retreating, or moving a piece for no reason then you've lost a tempo. That's how I think about it anyway.

tempo it's the italian word for time. so it's related to the time concept. basically a tempo it's the "time" required to make a move. it may be happen more frequently with intermediate moves and checks 'cause intermediate moves oftenly force your opponent to do somenthing and therefore they gives you an extra "time" for playing another move.

Yes Philip (MoxieMan) and I would add -- you don't count tempo and you can't gain it -- you only lose one or more tempi when you play in n moves what could have been done in n-1 or n-2 etc. moves
In endgames you often return a piece to a previous square, just to gain an advantage, that is not losing a tempo in my opinion.
In pawn endings it is good to have a tempo-move somewhere on the board to force your opponent to do something he doesn't want to do.
what is tempo? I've heard it mentioned but don't know what it means exactly. it related to check right?