So in WoW, every living being that dies goes to the Shadowlands, a separate plane of existence. Their souls are taken before a being called the Arbiter, whom experiences every aspect of their soul in an instant and sends them to an appropriate afterlife. Recently something has incapacitated the Arbiter, which has sent every soul that has died throughout the multiverse to The Maw, basically WoW Super Hell. Souls have power in the Shadowlands, called Anima, and all the souls flooding into the Maw has allowed the Jailer to harness their anima and make him, and by extension Sylvanas, far more powerful.
Sylvanas has allied with the Jailer because she believes that he will be able to tear down the system that the Shadowlands established. She claims that they will finally have choice, free will, even hinting at being able to choose who they love?
Anduin also states the obvious: "What makes you believe you're not just a weapon to achieve his ends?" which Sylvanas just ignores.
So, here's what I believe will be revealed: When Sylvanas killed herself ontop of Icecrown, she was sent to the Maw. Not because she was actually destined for it, but because the Jailer saw potential in her and intervened. She experiences terror and torture in the Maw and feels that her fate was completely undeserved (which, at that time, I would agree with). The Jailer comes in and is like "Sucks, huh? But if you help me you can avoid this", you know, lying through his teeth. So she makes a pact with him and has since set out to get this whole plot in motion.
At this point it seems that they're going to go the bullshit redemption route: Jailer backstabs Sylvanas, Sylvanas realizes the error of her ways, helps the Champions of Azeroth to defeat the Jailer, and then her and Nathanos probably fuck off into some corner of the Shadowlands never to be seen again.
Now, hopefully they don't go this route because it would be incredibly unsatisfying for all involved and, really, would just be the biggest fucking cop-out. Maybe Sylvanas is actually planning to turn on the Jailer, or something.
Either way, Sylvanas can eat the biggest bag of dicks.