“Don’t judge a book by its cover.”
Meaning: Do not judge a person (or thing) based on their appearance or what you see on the surface. In the same way that you have to read a book in order to have an informed opinion on it, you need to get to know someone (or something) better, deeper, before judging them. People are full of surprises, and often, what you see is not what you get.
This can be used when people jump to their own conclusions, based on prejudices and stereotypes, about somebody, before getting to know them better. For example, a homeless man who looks like he has not bathed in weeks, can be assumed to be uneducated, yet once spoken to, discovered to be a former university lecturer who suffered misfortune. By presuming he is uneducated, you would have judged a book by its cover.
Such assumptions are made constantly in our daily lives, and it does help to remind each other once in a while, not to judge a book by its cover.