A Microsoft file hosting and synchronization service.
I have the same problem on my Mac running 10.14.6 with the latest OneDrive 20.084.0426.0004
My OneDrive for business contains 400GB+, of which only 130Mb is actually downloaded (the rest is offline through Files On Demand) yet the MacOS (About this Mac / Storage), while reporting that 45GB is available on my 500GB drive, shows in its graph 512GB of Documents, plus all the other stuff (apps, mail, iOS, etc).
It's all looking VERY confused!