The Forza Rewards program tracks your progress across multiple Forza Motorsport games, starting at Forza Motorsport 2 and extending to recent games like Forza 4 and Forza Horizon. In-game progress translates to a universal pool of points, which corresponds to several tiers of rewards that can take the form of in-game currency, tokens and cars (also in-game). You're welcome to compare your progress to other players as well, but we don't recommend obsessing over tier pressure in a game like this.
If Forza's cross-game consistency sounds familiar, it's because of Microsoft's Halo Waypoint, a similar (though not as actively rewarding) program that spans the space-faring series. Just think of it as trading the lore for, umm, DeLoreans?