It's an acronym for Rainbows
. It can be used sarcastically or seriously. Sarcastic use includes when a situation or expectations are unrealistic and one has to put on a happy face regardless.
It can also be use ecstatically to describe an especially happy state of being within one's self about something wonderful.
Sue: The boss gave me this 1500 page report to review and edit by the end of the day. That's in 15 minutes. That dipshit thinks it's all RBUs.
Jenny: At least you gave good face.
Sam: How is it going with that fireman you are seeing?
Michael: It's all RBUs. I want to have his babies. LOL.