What does it mean? :confused:
In-character is used in the role-play forums and sometimes in the UN. It basically means that the person is writing using a role-played persona, as distinct to their own personal views and opinions (often referred to as out-of-character, or OOC).
Right now I'm speaking OOC: Out Of Character. If I started paying you drunken compliments, I'd (probably) be speaking In Character (IC) as Dicey Reilly, the amorous witch who is my nation's reluctant Co-President.
It's another way of saying 'pretending' or 'role-playing'. You're IC (In Character) when you're pretending to be a particular kind of person. You may be tolerant, relaxed and friendly, but you may find it fun to play a suspicious, touchy, narrow-minded sort. Or you may be a 21-year-old male, but in a particular story you've chosen to play the part of a frightened five-year-old female.
In-character posts are usually found in certain forums, such as Nation States and International Incidents. In the United Nations, some do, some don't; anywhere else, just be yourself. That is, be OOC (Out Of Character).
You'll sometimes see "OOC thread" in the story-telling (role-playing, or RP) forums. That's where people discuss what's going on in a story they're part of (like, "Your guy can't shoot him, he's already gone. Didn't you read the bit where he took off on a Harley?").
Read a few threads that interest you and it should come clear.
Edit: Or read Enn's post, since he's always clearer amd briefer than I am.