A Microsoft file hosting and synchronization service.
Update Feb 2, 2024. Same problem for me today. Totally freaked out and lost 4 hours of productive time as thought someone was accessing my files.
Spent 90 mins with MS Support chat which was useless. Told me to disable show recent files in quick access. Totally missed the point and ignored root cause. Zero care factor.
Very thankful for Markz&, though I unticked "show files from Office.com" If I hadn't found this thread I would be incurring $ for external IT support to ensure my laptop was not being remotely accessed.
Thank you MS community and Microsoft do better!