This proverb is a reminder to not judge something or someone based on their outward appearance. It encourages people to look beyond the surface and consider the true nature of a person or thing before making a judgment. It is a reminder to not be so quick to judge and to give everyone a chance.
From Wikipedia
The English idiom "don't judge a book by its cover", also known as "never judge a book by its cover", is a metaphorical phrase that means one should not judge the worth or value of something or someone by their outward appearance alone. For example, "That man may look very small and insignificant, but don't judge a book by its cover – he's a very powerful man in his circle".