The question that dominates the minds of most Star Wars fans nowadays are the origins of Rey, and who her parents might be (my money's still on that Kenobi bloodline), but Rey isn't the only one with a mysterious checkered past. Finn similarly has an unknown background, having been taking as a child soldier by the First Order. He doesn't even know his real name! In addition to exploring Rey's past, Star Wars: The Last Jedi will also shine a light on the origins of Finn and where he comes from.
We will learn more about his past and where he came from, and potentially why he made the decision [to escape] that he made. I'm also very curious. The question that needs to be answered is why he decided to leave as a stormtrooper in the first place. We will find out just a little bit more about him.
Star War: The Last Jedi has no shortage of backstories to get through. In addition to Rey and Luke Skywalker, the sequel will also tackle Finn's origins as a soldier for the First Order. Finn will be grappling with what he wants in the movie, and whether he wants to stay with the Resistance or not. John Boyega told Entertainment Weekly that Finn's decisions will be giving more depth, and fans will learn more about why he chose to leave the First Order. It was assumed that it was because he wasn't down with murdering innocents, but it sounds like it might be more complicated than that.
In The Last Jedi, knowing your past may just be the key to navigating your future. Rey will (hopefully) finally learn about her parents, and while that won't change who she is, it might help her to better understand her choices in her future. Finn may get to experience something similar, as the question of who his family is will be touched upon in the movie. When asked about Finn's family, here's what John Boyega had to say:
Yeah, definitely. But it's not explored in depth in Episode VIII. But he definitely has a past that is troubled. ... I don't know how all that's going to play out.
As to how his past is troubled, we likely won't know the answer to that for some time. I doubt that the identity of Finn's parents will be quite as much of a bombshell as Rey's will be, but that doesn't make it any less important to Finn (and us). It sounds like the idea will be introduced in The Last Jedi and then explored deeper in Episode IX.