To form an opinion about something based solely on its outward appearance.
It's a warning against making assumptions or judgments without considering the substance or content.
For example:
* Someone might "judge a book by its cover" if they think a person is intelligent just because they wear glasses.
* A company might "judge a book by its cover" if they hire someone based only on their resume, without interviewing them.
The proverb is often used to highlight the importance of looking beyond appearances and considering the inner qualities of something or someone.