It's an acronym for Rainbows, Butterflies and Unicorns. 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.
Sarcastic use:
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.
Ecstatic use:
Sam: How is it going with that fireman you are seeing?
Michael: It's all RBUs. I want to have his babies. LOL.
RBux is a free robux rewards website, it's kinda good but the restocks take forever. This time the restock is a myth, RBux will never restock. They keep telling us they will restock but every day passes and nothing happens...
Friend: Yeah, you can count on me, I will be coming tomorrow Tomorrow:
Me: Oh c'mon, you didn't have to lie like that, this isn't the RBux restock...