In character (IC)

In character (IC) means acting and speaking as your fictional persona rather than as yourself — the foundation of live-action role-play.

In character means that everything you say and do comes from your fictional persona rather than from you as a real person. When you are IC, your character's name, history, knowledge, and emotions are what drive how you talk and behave. If your character has never heard of a smartphone, you act as though the concept does not exist, even if your own phone is sitting in your pocket. Most games mark when play starts and stops with a call such as "time in" or "game on". Anything that happens while play is running is part of the shared story. Stepping out of it, to ask a rules question or check on someone, is going out of character (OOC), and your game's safety brief will explain how to do that clearly.