Appearances Can be Deceptive
Do not Judge a Book by its Cover
Both of these phrases have the same meaning. They imply that appearances do not always convey accurate information. That house sure looks beautiful on the outside, but appearances can be deceptive. What did the inspector say about the foundation? Sure, she seems nice, but do not judge a book by its cover.
Things can look different from the way they really are. Edward seems like a very nice boy, but appearances can be deceptive. Jane may look like she doesn't understand you, but she's really extremely bright. You must not judge a book by its cover.
These phrases are meant to tell people that you should not judge something simply by looking at it.