If something is decipherable, you can figure it out. Your dad's handwriting is terrible, but with a little work his note is decipherable.
When you need to untangle clues or decode mysterious messages, you have to decipher them. If this task turns out to be possible, you can describe the messages and clues as decipherable. When you eavesdrop on a conversation, you're hoping for some decipherable conversation (and some good gossip). And if you're writing a post card to your friend, the post office can deliver it only if the address is decipherable.