Your opponent had a pretty good idea what you might play next (either because you were in check, or would make an obvious capture, or because it was a simple opening position, etc.) and decided to pre-program his or her response.
I find conditional moves especially helpful if I spend time analyzing a position deeply and don't want forget later what I had planned.
how do you pre-program your response?
When it is your turn in an online turn-based game, a clickable box is available that says, "Conditional Moves." When you click there, a popup window will open. You play the move that you think your opponent will make. Then play your response. You can continue on that line or start a new one.
If you want to try it out, you could start a new game where you play as black. Pre-program your moves in case your opponent plays 1. e4 or 1. d4
In my online chess games this has occured a few times. What does it mean?