Our ancestors create many proverbs that can be used in all situations. All that glitters is not gold is a proverb that means about the general truth about the character of mankind. Glittering is the feature of gold. And it very precious and costly. All women have craze for gold ornaments. They choose gold ornaments in the functions. And evaluate the others design of ornaments. It shows the characteristics of feminine. But the proverb means that, don't believe that all the items that are glitters is gold, there are different items that have glitters. Firefly have glitters but it is not gold, fire sometimes have glitters but it is not a gold. But gold have it's own glitter. Only other can imitate it. We have imitate gold, that have a glitter covering of gold but it is not a pure gold.
This proverb can be used to the human character. There are some people who tries to pretend to be a good person. But they are like a imitate gold. After the few usage their outer face get breaks and shows their real face. We used to give it as an advice for the people who blindly believe everyone.