The English idiom “don’t judge a book by its cover” is a phrase that means one should not judge the worth or value of something by its appearance alone.
Let me tell you a story:

A Fox one day spied a beautiful bunch of ripe grapes hanging from a vine trained along the branches of a tree. The grapes seemed ready to burst with juice, and the fox’s mouth watered as he gazed longingly at them.
The bunch hung from a high branch, and the Fox had to jump for it. The first time he jumped he missed it by a long way. So he walked off a short distance and took a running leap at it, only to fall short once more. Again and again, he tried but in vain.
Now he sat down and looked at the grapes in disgust.
“What a fool I am,” he said. “Here I am wearing myself out to get a bunch of sour grapes that are not worth gaping for.”
And off he walked very, very scornfully.

This is why we should not judge a book by its cover. If the fox tried a bit more to reach the grapes then he could have gotten them instead he gave up and thought the grapes were sour. If the fox would have tasted them then he could have judged if it was sour or sweet.