A Microsoft file hosting and synchronization service.
Clearing out your OneDrive storage instead of purchasing more storage involves turning off the backup in the Manage Backup settings and returning your files, which had been synced to OneDrive, back to their default folders (on your PC only) in File Explorer.
Deleting OneDrive files should not be done until the folder backup is turned off and your files returned to their default folders in File Explorer. If you leave the folder backup on, your Desktop, Documents, and Pictures folders will once again be repopulated in OneDrive.