What is the meaning behind the proverb "not all that glitters is gold"?

At it‘s most basic level, this quote expresses the idea that something’s appearance is not necessarily a true representation of what it really is. In other words, things are not always what they appear to be.
A deeper analysis would show that this proverb also implies that something’s appeal (glitter) in fact may be hiding a negative or unpleasant reality (not gold after all). Gold is used here because we associate this precious metal with a high level of value. Though something else may mimic the glittering of gold, it doesn’t mean that it will hold that same value.
This can be applied to actual objects (such as fake Gucci glasses), people (someone who is handsome but then is discovered to be a liar or cruel to others), or even situations (a job promotion that seemed great but comes with far too much work).
In order to truly appreciate this proverb, one must realize that not only must they understand what phrase means, but also the lesson it is conveying. The warning “not all that glitters is gold” implies that a person much look deeper than the surface and not make assumptions based on first appearances. Critically analyze the situation or object/person and don’t make hasty decisions about them. In this way, one can appreciate both the meaning and lesson of this proverb.

The popular English expression "Not all that glitters is gold" is used to mean that sometimes things that look valuable, attractive, or promising turn out to be anything but. At an extremely literal level, one example of this can be found in iron pyrite, otherwise known as fool's gold, which is a mineral that at first glance may be mistaken for actual gold but, upon closer examination, is relatively worthless. An interpersonal example might be if you met someone who was extremely attractive and seemed superficially nice and interesting, only to find out upon spending more time with them that they were actually cruel and boring. Beauty on the outside is not the same as beauty on the inside.
To think of examples from your own life, try to remember a time when you had high expectations for something based on cursory knowledge of what it was only to dig deeper and be disappointed, or a time when you've been tricked by something's outward appearance. We've all been there!