Quick Resume is a feature unique to the Xbox Series X that allows players to switch between multiple games almost instantly. The console uses its custom SSD to save the state of several games in suspended mode, even when you turn off the console or switch to another game.
When you return to a game, it picks up right where you left off, eliminating the need to wait for loading screens or start from a save point. Here is some information on how to disable the quick resume settings.