Definition
To make judgments about things or people by considering external appearances alone.
See Also
How People Actually Use It
Don't judge a book by its cover basically means that you shouldn't judge before you know, you should't judge because 9 times out of 10 the reality is different than you think
"Old dude: He has dreads and tattoos, he must be dangerous! Youngin: Don't judge a book by its cover."
Source: Urban Dictionary