Definition
An unplayable shot is a situation where you have found your golf ball, but you determine it is not playable and intentionally take a penalty stroke and drop the ball in a spot that is playable. For example, if you hit the ball and it gets stuck in a tree, you would call that "unplayable". You would drop the ball next to the tree, take one penalty stroke, and continue playing.
Usage
I found my ball in the tree, but I'm going to declare it unplayable and drop with a penalty stroke.