Mask part of a video
A mask composites one layer through the shape of another, which needs layers and therefore a timeline. The full editor has masks — this page takes you there with your file.
Masking is about what shows through what. That question only arises once there is more than one layer.
The editor supports shape masks with soft edges, which can be keyframed to follow something across the frame.
If what you want is simply to hide part of the picture, the blur and mosaic tools here do that in one pass without a timeline.
FAQ
Did you mean hiding something?
If so, the blur or mosaic tool on this site does it directly and faster.
Can the mask move?
Yes, masks can be keyframed.
Are the edges hard?
Soft edges are adjustable.
Can I invert a mask?
Yes, in the editor.
Is it free?
Yes.