"Don't judge a book by its cover" what does it mean?
It means that if you see someone doesn't try to judge that person without knowing him/her.
Judging people is not a good deed at all. But when we are with a person for the first time then we instantly think about the things about that person that comes to our minds.
Is the person pretty? Is the one polite or not? From appearance to the character we go on and on. But the expression tells us not to judge people before knowing anything about them.
Some people may look angry or upset and by looking at their faces we may think to ourselves that they are mean or unfriendly but after getting to know them better we may realize that it is only temporary and they are suffering from a problem or something.
On the other hand, some people may look happy and we think that they don't have any problems but after some time we realize that they have many problems in their lives but they try to hide them behind their smiles and happiness.
All in all never judge a book by its cover is used for this kind of judging people. When you meet someone for the first time try to know that person without any judgment and before hands ideas. Try to have an open mind about them until you know them good enough. Even then don't try to judge them because judging is not an easy job at all.
If you don't like to be judged it's better not to judge others either.