You can't judge a book by its cover means that before you can judge something, you need to take a deeper, closer look at it. Example: "He dresses in plain clothes and drives an old car. Who would know he is the richest man in town?" Reply: "You can't judge a book by its cover."

The value of something is not always obvious from what we see on the surface, so we should save our judgments until we have more experience. Example: "That professor may seem slow and awkward, but don't judge a book by its cover. He is one of the brightest minds in his field."

You can't judge a book by its cover means that when you have only seen the surface of something, you cannot know what is on the inside. Example: "Don't go falling in love with that girl so fast. Wait until you get to know her better. You can't judge a book by its cover."