As others have said, the sentence is not in modern English. It comes from a Shakespeare play written in the 1590s. (A very similar saying also occurs in J.R.R. Tolkien's fantasy series Lord of the Rings, in a poem that begins, "All that is gold does not glitter.") In the sentence "all that glitters is not gold," the word "all" essentially means "all things." The sentence as a whole means "It is not true that all things that glitter are gold." Some things may look valuable (they may "glitter"), but in reality, they have no value (they are "not gold"). They look like gold, but they are actually worthless. Tolkien's line from Lord of the Rings conveys a similar message, but reversed: "All that is gold does not glitter," meaning "Not everything that is gold glitters" (some gold things do not glitter). Some things are valuable but do not look valuable. You cannot always tell whether something is valuable just by its appearance.

The quantity is not important here. When we say "all that glitters is not gold" what we are really saying is "be careful'! If you've ever heard of "Fool's Gold" you'll understand exactly what I mean. Fool's Gold glitters like real gold, but it's not worth anything!

It is not the plural glitters, it is the third person present form

The quote from Shakespeare (The Merchant of Venice Act II, Scene VII) is actually 'All that glisters is not gold'. So if you want to be irritating you can point it out whenever anyone mentions it! :-)

"All that glitters is not gold" is an old fashioned phrase that doesn't follow modern patterns, so do not worry about struggling with it! What it really means is "some things that look like gold, are not really gold". It is a proverb about not blindly accepting the value of things.