Requests a takeback of '#' half-moves in the current game. If '#' is omitted then 1 is assumed. A "half-move" is defined as a move by one player. To request that the last move made be changed (whether you were the one to move last or your opponent moved last), you would request "takeback". If you wanted to change the last move made as well as the move prior to that, request "takeback 2".
Here's an example: suppose you are playing White and your opponent has just placed a piece on a square where it can be easily captured, and you capture it. Your opponent might realize the weak move he/she made and request "takeback 2" so that he/she doesn't lose material so easily. If you wish, you can accept the "takeback 2" request and continue the game from the previous position in the game. Both moves would be reversed, and it would be Black's move to make.
See also: accept