OneDrive for Business and the local disk

I always thought that OneDrive for Business was something like a remote disk, the things you put there do not burden the local disk. But when I ask for the properties it tells me that OneDrive For Business is located on C, and when I drag files from C to One Drive for Business, the space on the local disk decreases instead of increases. I apparently have done something wrong, but what?

When I check online, most of the files are there, except some that are probably still synching.

Have I misunderstood the workings of OneDrive fB?