This mask is defined through a polygon and a feather parameter.
To edit the points of the polygon, you need to use the
Edit Mode of the
Viewport:
The
Edit Mode is relevant to the currently selected element in the
Element Tree, so take care that the polygonal mask is currently selected.
Now, press
[CTRL]
and click in the
Viewport to create points:
When there are more than two points but the polygonal mask is not closed, it will display a 50% overlay on the whole image.
To close the mask,
[CTRL] + [CLICK]
on the first point:
The points of the polygon can be moved at anytime by dragging them in the
Viewport.
The only necessary thing is to be in
Edit Mode with the polygonal mask currently selected.
To animate a polygonal mask, just drag and drop the "polygonal mask points" parameter inside of the
Timeline. Now each key you'll add on this parameters will concern every points of the mask :
You could also animate all the mask by using
Exposed Parameter on the "Points" parameters.