Roleplaying is basically making up a fictional character and pretending to act like that character. It is usually performed in MMORPG-s but you can do it in pretty much anything.
Example: You decide you want to be a tank in Terraria and thank mobs to protect other players from harm, so because of that you talk in such manner, and you do everything in your cause to save the players, while saying thigs like "I shall protect you, stand behind me friend!" or "While I'm by your side, nothing will hurt you!".
Sounds little weird when I put it like that I guess but I think you get the meaning of it.