A Microsoft file hosting and synchronization service.
Quoting the video in this article:
https://support.microsoft.com/en-us/office/save-disk-space-with-onedrive-files-on-demand-for-windows
To make sure a file is always available, even when you're offline, right click it and choose "Always keep on this device".
If you want your Documents folder backed up you will need to turn on the OneDrive PC Folder Backup for the Documents folder.
That would place your Documents folder in the path of C:\Users\Sara\OneDrive\Documents. You don't have to open your OneDrive folder to get to your Documents. Just open the Documents folder as you have always done so in This PC.