When used with gradable adjectives (e.g. good, bad, tired etc), quite means ‘fairly’ or ‘rather’.

The film was quite good, but it could have been better.

With non-gradable adjectives and adverbs, quite means ‘completely’.

It is quite impossible. (= It is completely impossible.)
You are quite right. (= You are completely right.)

Compare:

She speaks Spanish quite well, but she has got a strong Canadian accent. (= Her Spanish isn’t perfect)
She speaks Spanish quite perfectly. (= Her Spanish is perfect.)

Quite can be used to modify verbs, especially in British English. The meaning depends on whether the verb is gradable or not.

Word order with nouns
Quite can be used to modify nouns. Before a noun with a gradable adjective, we use quite a/an.

She is quite a brilliant girl.
It was quite an interesting film.

Before a noun with a non-gradable adjective, we use a quite.

It was a quite perfect day.

Quite cannot be used before comparative adjectives or adverbs. Instead, we use other degree modifiers like rather, a bit, much etc.

She is rather older than her husband. (NOT She is quite older than her husband.)

Note that the expression quite better is an exception to this rule.

Not quite means ‘not completely’ or ‘not exactly’.

I am not quite ready. (= I am not completely ready.)