It's when you pretend to be someone or something you aren't. Like cosplay, a little. Votary to popular belief not all text-based roleplay is sex, and many role players only want an interesting experience developing a character and creating a story but are swamped by pervs who just wanna cyber...
It's basically a unique and often very understood sort of storytelling, and go almost anywhere other than ATA games (forums, chatrooms) to roleplay and you will find tons of mature people not interested in sex roleplay.