A Microsoft file hosting and synchronization service.
I'll be more polite than most of the replies, but this is a worthless answer.
Unlinking and re-linking my computer is not a solution. That might be the answer if OneDrive has stopped syncing entirely and every other option has failed. But it's not the answer to force a sync-on-demand. It would take hours for that to re-download all my files.
And the "onedrive.exe /sync" suggestion doesn't work at all. It opens up an Explorer window to my OneDrive folder, but nothing is updated.