In the main threads you are required to stay true to the events of the timeline.
However, you can create your own threads based off a story you would like to roleplay with another member.
I believe character's posted in the "Character Profile" section are required to be true to the timeline as well, but if you have an idea for an OC based off of say, The Clone Wars, you could post it in Player Workshops and RP with someone who wants to do that era.
You can also start your character off in Player Workshops so that you don't have to finish it all at once, and then get one of the staff to move it to Character Profiles.
Hope I'm not dragging on too much. XD
Looking forward to meeting you.
P.S.
Unlimited cookies await you in the Dark Side.
The overall plot for the site follows the main story (found here), and is usually moved forward using Main Threads on the Story Board (see: these examples). While those threads are designed to move the overall story for the site forward, you are free to weave your own character's story however you like (be it participation in a Main Thread, or just simple RP with a partner in an ASK or OPEN thread).
To momentarily touch on what Dark Child has said:
Yes, all characters you post for intended use in the RP must match up with our current timeline (~1,030 BBY). That said, we do have a Diverse RP section where you are free to roleplay whatever you like. Halo, Warhammer, other Star Wars eras, people have even done Harry Potter RPs there before. So if you'd like to change it up every once in a while, the Diverse RP board should meet any needs you might have.
If you require any further assistance or have anymore questions, feel free to ask.