The Hartmann mask and Scheiner Disk are simple devices for making focusing a lens easier. They require that you target is a point of light, such as a star or distant planet, therefore are no good for day-to-day use.
A Scheiner Disk is a mask with two holes, whereas a Hartman mask has many holes (typically three).
The mask is like a lens cap with holes in. The holes are positioned in such a way that when out of focus, they cause multiple diffraction images as shown in the first image. As the lens is focused they merge into a single point, and then beyond focus, they separate again.