The phrase is often used to describe people who are overly talkative or boastful, but whose words are ultimately meaningless or empty. It can also be used to describe people who seem to have a great deal of knowledge about a subject, but upon further examination, it becomes clear that their understanding is limited. In everyday speech, the phrase can be used to describe someone who is not as intelligent or wise as they seem.
The phrase is a warning against being fooled by appearances. Just because someone seems impressive on the surface does not mean that they are actually well-versed in any given subject. It is important to look beyond surface-level attributes to truly understand a person's skills and knowledge.