ahh RP, brings back old high school memories lol .
So basically in RP'ing, it's like you take a story, and you become the main character. Normally a setting is set up so, (imma use KaiSoo pairing) one day Kai walks into the cafe where Kyungsoo works.

So you and the other user, one of you will be Kai and the other will be Kyungsoo, then you act it out from there over the internet.

So it could be something like:

RP1(Kyungsoo): welcome to the DoReMi cafe! How can I help you sir?
RP2 (Kai): hello, I would like to order you with icing on top ;)
RP1 (Kyungsoo): /blushes deep tomato red/

LOL that's just an example, but I hope you can get the gist of it .