A typical in-between move happens as follows. The opponent captures something, for example takes a knight with a bishop to exchange pieces. Then instead of taking it back, you check the opponent's king with your rook. Then the king has to move and forfeit its right to castle. Then you re-capture the bishop with your queen. Another example. Your knight has just been threatened by a pawn. Instead of moving it away, you threaten the opponent's knight with a pawn.
In-between moves can improve your position, win material and lead to successful mating attacks. Searching for in-between moves when looking at positions and calculating variations will broaden your chess vision. It is a powerful resource and is used on everyday basis by strong players.