Rather

Please who can give me an exactly explanation of the word RATHER.
Thanks!

It's an adverb. It can be used to indicate preferences:

-Do you want fish or meat?
-I'd rather have fish.

-Should we go to the movies tonight?
-I'd rather not. (I'd rather not go.)

And it can be used as a mild intensifier.

-This is good.
-This is rather good. (A little stronger, possibly indicating that something is unexpectedly good.)
-This is very good. (Stronger still.)

Overuse of 'rather' as an intensifier is poor style, though. Use it rarely.