Meaning that not everything which is shiny and superficially attractive is valuable. This can apply to people, places, or things that promise to be more than they really are. The expression originated in or before the 12th century and may date back to Aesop.
Shakespeare is the best-known writer to have expressed the idea that shiny things aren’t necessarily precious things.
All that glitters is not gold;
Often have you heard that told:
Many a man his life hath sold
But my outside to behold:
Gilded tombs do worms enfold.
Had you been as wise as bold,
Young in limbs, in judgment old,
Your answer had not been inscroll’d:
Fare you well; your suit is cold.
Many different ways of expressing the idea that ‘all that glitters is not gold’ were in general circulation well before Shakespeare’s day and it was a common enough notion to have been called proverbial by the 16th century. For example, the 12th century French theologian Alain de Lille wrote “Do not hold everything gold that shines like gold.” Geoffrey Chaucer expressed the same idea in The House of Fame, 1380: “Hit is not al gold, that glareth.”
John Dryden published a poem in 1687, called The Hind and the Panther:
For you may palm upon us new for old:
All, as they say, that glitters, is not gold.
Here is one more modern example of the universality of the phrase. Panning for gold often results in finding pyrite, nicknamed fool’s gold, which reflects substantially more light than authentic gold does. Gold in its raw form appears dull and does not glitter.
But some things do glitter that perform the same function as gold. I was given a brand new loonie in change yesterday and it shines like a miniature sun.