This English idiom can be more commonly heard in social situations rather than professional environments and is a metaphorical phrase that means one shouldn't prejudge the quality or value of something by its outward appearance alone. One should not form an opinion on someone or something based purely on what is seen on the surface, because after taking a deeper look, the person or thing may be very different than what was expected. Literally, if one was to judge a novel based on the cover image, then there would be a lot fewer books read in the world.
My son never wanted to eat watermelon because he didn’t like the exterior green colour. I cut some up and gave it to him without him knowing what it was, and he loved it! I made sure to tell him, “you should never judge a book by its cover”.
The woman appears to be insignificant sitting in the meeting room, but don’t judge a book by its cover, she’s the CEO. She likes to take in everyone’s input before she says anything.