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: