Don't judge a book by its cover.When we enter into a book shop to search for a good book and buy, we will get attracted by the cover of the book. Sometimes we will not even open the book with a plain cover. We may decide whether a book is for us or not by seeing the cover. Those times we may be missing a book simply because the cover appears plain . However, if We open that book and see what is there inside we might have got impressed with the book.
This is also applied to people. We will tend to decide on a person by seeing the physical appearance. However, if we really want, we should speak to him,understand him and see what's he inside. Some time we will end up with an entirely different opinion than what we had when we have seen him. So this proverb is used either as advice or a warning that a person should not be judged by what they appear outside.
So let us not simply go by the physical appearance.