If something is imaginable, it's something you can think of or conceive. Often, this means the same as "possible."
If something is imaginable, then it's possible to conceive it or grasp it with your mind. Since the mind is powerful, a lot of things are imaginable. This word is often used in exaggerations or to make something seem impressive. People will say things like, "That was the worst day imaginable," which means it would be impossible to think of a worse day. If you can think of something, it's imaginable.