A Microsoft file hosting and synchronization service.
To ensure a local copy of your files is kept on your PC and sync the files to OneDrive as a backup, right click the OneDrive cloud icon on your Taskbar and select the OneDrive 'Help and Settings' icon, then choose Settings.
Select the 'Sync and back up' tab.
Expand the 'Advanced settings', and toggle off the 'Files On-Demand' setting.
Then wait for OneDrive to download all your files from the cloud to your PC, do not interrupt that process.