A Microsoft file hosting and synchronization service.
I suggest you set OneDrive to start automatically when you log in to your computer.
To avoid "Processing changes", I use Files On-Demand with most of my files freed up. But I also make periodic backups to a local external drive just in case I should ever lose access to my account.
https://www.youtube.com/watch?v=UzIA9uykIhE&t=43s
You can free up all your files by going to OneDrive settings > Sync and backup > Advanced settings, then scroll down to Free up disk space.
You can keep your files freed up in Storage Sense: https://www.youtube.com/watch?v=9yKbOxn4E0A