ANSWER:
The layer mask allows you to hide parts of a layer without ALTERING any part of your image. This gives you a lot of FLEXIBILITY and causes no permanent damage.
EXPLANATION:
A layer mask is an important tool in image editing software like Photoshop that allows a user to modify the transparency of the selected layer. They make use of three colors: white, black and grey.
When a layer mask is in white, it is said to be 100% visible, and when it is in black, it is 100% transparent.
:) hope this helps! :)