RP stands for Role Play (as you probably know.) Basically, the leader picks a topic. Say, Warrior Cats. They make a post reading "Warrior Cat RP!" or something like that. In it, they will include a form for entries, rules, plot, etc.
Soo, for example.
Plot: Tigerstar comes back from the dead and the Clans are thrust into chaos!
Rules: Medicine Cats cannot have mates or kits
Follow the Warrior Code
Follow the board rules
When it is time to Role Play, or make POVs (Point of View), people who entered will make a story based on the plot in their characters point of view.
Here is a quick example of a POV in Firestar's point of view:
"Quick, Firestar, come!" Graystripe called to me.
I padded over to him, my ginger fur rippling in the breeze. Graystripe's eyes were swimming with worry and fear. What could it be this time?
"Tigerstar is back from the dead!"
It would be much longer and a little more descriptive and exciting, but that is as best as I could play out a quick POV.
If you don't know what Warrior Cats are, they are cats--of course--that live in four clans, each with a leader and a deputy (basically a co-leader) and many more rankings. The books are by Erin Hunter and they are really good. You should read them.
For more information, check out some existing Role-Plays and explore the possibilities! Your imagination is endless, and all ideas are respected around here, so you can start a new trend of topics to Role-Play in.