OOC is an acronym for out-of-character. This refers to everything that is not considered of roleplaying material. OOC refers to anything involving syntax, coding, real-life discussions, etc.
The OOC command will allow you to discuss real-life subjects without cutting into the roleplaying experience too much. You can imagine if you are discussing such things as writing a book on the history of the world with another player, and then right in the middle of the discussion saying something like, "Time for dinner! I have to go!" If the other player was very into discussing the book, they might get confused about your comment.
This is where the OOC command comes in. It will simply put "(OOC):" at the beginning of your message and automatically show everyone in your room that you meant that to be an out-of-character comment.
Although OOC comments are looked down upon by some players, in that it will ruin a good roleplay, it is not going to be banned completely. The OOC command is there for you. We encourage you to use it, but you don't have to. Of course, be respectful to those who might wish you to utilize it.