In character, shortened IC, is a roleplaying term. Whenever you are roleplaying, you are "in character", meaning you act as your character would in that situation. Behaving as you, the player, normally would is termed out of character, or OOC.

Out of character comments are frequently marked by using the emote chat system.

In character: Mina glares at Throg. "That's because I hate you."

Out of character: Mina - brb, dinner.