The Automatic Quit feature will shut down Scrivener automatically after a certain period of inactivity. This is especially useful for preventing conflicts and version issues if you often walk away, forget to close Scrivener, and then try to work on a project on another device.
The default inactivity period is set to 30 minutes. You can adjust the time period and turn the feature on/off under File>Options>General>Startup.
When Scrivener shuts down automatically, it will backup (unless the project is excluded from backups) and close all open projects. When you launch Scrivener again, those same projects will reopen.