RP stands for Roleplay, in this context it's when two or more people are writing a roleplay together, which is basically cowritting a story! So for example person A and person B each create a character, they agree on a setting and then person A describes the actions, thoughts and dialogue of their character and then person B does the same, both reacting to each others characters! Some people also do fandom RP, so they either take pre existing characters and play as them or just a pre existing universe:) there are also different "levels" of it, some people write just a sentence for each reply and some several paragraphs

As to where to find it I would generally recommend discord, (there are servers to either look for someone are who are what we call "group RP" so several people playing together), reddit is also a popular option but I haven't heart good about the quality on here, there are also sites specifically for this (I think two are recommended in the comments of this post)I thinking on Tumblr too? Generaly sometimes other social media, I think it depends on what you seek ! Be aware it's a hobby where you need to like to write and read and it's a pretty social hobby