Fair bit means "quite a lot" or "plenty"

A "fair bit" is a modest amount. I would say it is more than a "little bit". A "bit" is a small amount. Usually, we say "Give me a little bit. I'll try it." A "fair amount" means a moderate amount or a pleasing amount, not a stingy amount. He has a fair knowledge of English => He has a pretty good knowledge of English. Fair = halfway between bad and good => not too good but not too bad either. In the middle.

It means "quite a lot" but it is very colloquial.