Imagine your good old, comfortable, conventional "pick up coins and jump on the enemy's head" platformer game.
Now imagine that good old, comfortable conventional game has already been completed. Not by you. By someone else. Someone random.
And now you have to undo all those steps in their correct order - or you will cause time paradoxes.
Every single decision, smart or stupid, that was made when this game was originally played through - you have to undo. Revive all killed enemies, put back all collected coins, and give back the bosses their lives - one by one.