This isn't really a mask in the more commonly perceived digital graphics sense, in it's not actually creating transparent negative space around your image. The "masked" area is still a solid color, in this case white.

It would be more appropriate, and less flashy, to call this PNG matting, because it's far more analogous to cutting a matte (overlaying a paper stock frame over your artwork to shape it's borders) than it is to an actual mask (using an overlaid shape to actually define a new transparent alpha channel for your image)