OOC stands for "Out Of Character." It's when you're talking to the other RP participants about something outside of the actual RP storyline.
For example, if you need to get clarification about what's going on or let people know that you can't post for a few days, that's something you'd say as yourself (a person at a computer), not as your character.
In any RP, there needs to be some way of indicating which part(s) of your post are "out of character" and which are "in character." If OOC text goes within the same thread, it's often differentiated by placing the text within ((double parentheses)), //slash marks//, or something like that. But for the Serious RPs on the forum here, we allow for separate threads to be used solely for OOC text, so that everything in that thread will be out of character, and everything in the RP thread will be said in character.