A Microsoft file hosting and synchronization service.
I got some help from Microsoft and found that I needed the latest version of OneDrive on my MacBook which shows in Finder which files /folders are using disk space vs cloud (little cloud icons). Right click for 'Get Info' confirms cloud based OneDrive files are using zero disk space.
The problem occurs in Macbook Storage Management which recognises OneDrive files as using space when they are not. So don't go deleting files yet until you've got the new OneDrive for MacBook and do some checking. I tried to post a query on Apple support but it didn't go through today. Will try tomorrow.
I hope that's helpful.