The very common English idiom "don't judge a book by its cover"(common phrase)is a metaphorical phrases which means "don't determine the worth of something based on its appearance". It is probably the most common expression used in English to convey vanity.
So, from now on, please don't judge something by its shape or seeing something physically. If you have a new friend or meet a stranger, you have to know how the way she/he speaks, how the way she/he thinks. Essentially, don't see someone or anything by its physical. That's all from me. I'm sorry if my English is not good haha. Cheers!