Room - an individual chat forum within a chat site, or a stand-alone chatroom on a smaller site. A large popular chat site may have as many as 200 rooms, a small chat site may have one room used at different times for different chatters or less than 10 rooms used simultaneously. Rooms have names intended to inform the chatter of the interests of the chatters within them.
rofl/ROFL - rolling on the floor laughing. In reply to a funny message.
R - are.
RCI - rectal-cranial inversion; the person referred to has their head up their arse, metaphorically speaking.
Re - regarding; in reference to. Usage example: "LOL re the unicorn, julie."
RTFM - read the fucking manual.
RTI - real time interruption.
RUOK - Are you okay?