In the world of art, design, and 3D – there are so many kinds of masks.  With Illustrator, I came across two so far.  The first is a clipping mask.  This is a mask that works off of the object paths/shapes.

The second is an opacity or alpha mask.  This works off of the colors.  White means that the pixel beneath should be kept at 100% and black means 0%.  Came across a tutorial from Adobe here:

https://helpx.adobe.com/illustrator/how-to/illustrator-opacity-masks-transparency.html