When something's unclear it's hard to see or understand. If the directions for assembling your new desk are unclear, you might end up with two upside-down drawers and a handful of unused nuts and bolts.
If you don't explain something accurately, it will be unclear to the person who's trying to understand. And when a photograph is badly out of focus, it's also unclear — you can't quite see the details. You can also use this adjective for the feeling that results in these situations: "I'm unclear about exactly who's in this photo and why you wanted me to see it."