It is where 1 person often called the gm has a story in mind and some other players come in and take roles. Some roleplay, one player pretends to be Petrie, or one of the gang or other lbt character and they post as that character or characters. Some also do original characters they have made up themselves. The players and gm post and roleplay, the gm does the narrating, runs the bad guys, and often may rp as a character also.
Hope it was on ok explanation. You can look in the rp sections and read some of the threads to try to help understand.