An obligatory finesse occurs when declarer is "obliged" to duck an honor. For example:
Needing 3 tricks in the suit, you lead toward the king, which holds the trick. You continue with a low card from dummy, East playing low. At this point, you have an obligatory finesse. Play low from dummy and hope that West started life with the doubleton ace.
You lead low to the queen, which wins. A spade back to hand should be ducked in case West began with a doubleton ace.