Well, do you mean OCs in general, or OCs on MLPForums?
In general OCs have a lot of uses, from roleplay to fanfiction to art, and are simply characters created by you that are meant to inhabit a world created by someone else (like how pony OCs are meant to inhabit Equestria), more or less. They can be a version of yourself, or a completely independent character, depending on what you want to make.
On this particular website, people create OCs in one of the two character databases almost entirely for use in roleplaying. Only those made for the Equestrian Empire section need to be approved, those made for the Everfree Empire section don't need approval. In both cases, from there you can ask to join roleplays or make your own roleplays for other people to join, using your character's page to let people know ahead of time who the character you'll be playing as is and some of what to expect from them.
