It can be modulate an adjective, meaning something like "somewhat" or "a bit", as in "the book is rather long" ... or ... meaning not much at all, as in "that's rather nice, isn't it?"
It can also be used comparatively, meaning "in preference to", e.g. "I'd rather have dental surgery than have to listen to another presidential debate", or "more exactly", e.g. "We're cooking dinner for our friends tomorrow, or rather, my husband's cooking and I'm doing the washing up".