It is a way to make game go a little quicker. If you think you know what your opponent's next move will be, you can input that move and then input your reply. If your opponent makes the move you thought he would make, the system will automatically play your reply and then it's your opponent's move again. You can make a whole sequence of conditional moves with different variations.
It's a good way to make obvious or forced moves.
What do they mean by conditional move?