Short for 'Roger That Red Leader'.
A short, usually textual response to a well formed plan. It acknowledges that the person in control of the plan or situation has covered all eventualities and you agree with their conclusions.
The original phrase featured in 'Star Wars: A New Hope'
Your Bro: The party starts at 6pm, I've picked up the keg, you need to bring beer pong and arrive at the latest by 8pm.