Rewind in Forza is like a physics do-over. It’s only available in Free Play, Rivals, and Career (where the Assist is enabled); it’s not available in live races (multiplayer private/hoppers/leagues).
Turning Rewind to Off in the Assists menu for a race will provide a % credit payout boost for that race. Using Rewind in a race does not cause any financial or time penalties.
Pressing Rewind once will take you back (100 meters or so? Someone else may have to verify that). This enables you to change your inputs like steering and braking, but only to the extent that physics allows - if you are skidding when you press rewind and it takes you back to a point where you started to skid, you’ll be skidding when you come out of the rewind. You may notice that Drivatar behavior changes slightly after rewinding. Rewinding from a point of car damage can help you prevent the damage and therefore prevent the need to pit for repairs or just keep your car body clean for Photo Mode.
There’s no limit to rewind-play-rewind-play-etc but there is a limit to how many times you can hit rewind and thus how far you can go back (this might be limited by number of R presses or by track sector). The limit is somewhere less than a lap from your furthest progression in the race.
You’ll notice split times for your laps come up at various points on a track - these are sector points in the track (not visible) defined by the game. If you go off track or collide or draft or do something to create a dirty lap /!\ in the last sector of a track, then this would normally dirty the next lap automatically, and you’d have to go all the way around again to start a clean lap. If you press Rewind in the last sector after causing the dirty lap and go back to before the sector start point, and keep the last sector clean from there to the lap marker, your next lap will start clean.
As more users provide detail to this thread I’ll sum it up in the FAQ pinned at the top of the forum. If you can come up with additional specific questions (Does it do this? like other games? etc) that will help provide more comprehensive answers.