A mask limits an effect — or the clip itself — to one shape. The part people miss is that the mask lives on whatever you want to limit, not on a separate layer.
Steps
Select the clip in the timeline and open the Effect Controls panel.
To hide or reveal part of the clip, find Opacity and click one of the three mask shapes beside it: ellipse, rectangle, or the pen for a free shape.
Drag the handles on the program monitor to fit the shape to what you want. Feather softens the edge — a few pixels is usually enough to stop it looking cut out.
To mask an effect instead, drop the effect on the clip first (a blur, say), then use the mask shapes under that effect rather than under Opacity. Now only the shape is blurred.
If the subject moves, click the Track Selected Mask Forward arrow. Premiere follows it frame by frame and writes keyframes; let it run and then fix any frames it lost by dragging the shape.
Tick Inverted to swap which side of the shape is affected.
Worth knowing
Tracking is slow on long clips and works best on something with a clear edge. If it drifts, stop it, correct the shape on the frame where it went wrong, and track on from there. For a hard-edged mask over a moving subject, it is often faster to keyframe the mask by hand every ten or fifteen frames.
Faster than searching: box the part of your screen you
cannot work out and ask about it directly — in Premiere Pro or anywhere
else. Answers arrive where you drew the box.