A Microsoft file hosting and synchronization service.
If you don't want to use OneDrive, you should have a backup copy saved somewhere else. If you lose your one and only file, we can't help you recover it.
I tried to keep saving a different named version each time, but that got excessive.
It's certainly better to have these backup files, but you should also save copies on a backup drive.
OneDrive "autosave" keeps these backup temporary files stored in OneDrive for quite a while.