Roleplay on IRC is two or more people engaging in a fantasy scenario, acting out scenes or situations. The players create a character they choose to play as. It can be a great way to exercise your grey matter. Have you ever played an RPG video game? That's also roleplaying – it's in the name!
Roleplaying between adults on IRC usually involves sexual scenes, but it is certainly not limited to sex. Cybersex is different, in that it doesn't always involve character roles, and is mostly sexual in nature. See Roleplay vs Cybersex to learn the difference between them.