Masking

This property determines how Unity blends the lighting with the colors of the spriteA 2D graphic objects. If you are used to working in 3D, Sprites are essentially just standard textures but there are special techniques for combining and managing sprite textures for efficiency and convenience during development. More info
See in Glossary and other GameObjectsThe fundamental object in Unity scenes, which can represent characters, props, scenery, cameras, waypoints, and more. A GameObject’s functionality is defined by the Components attached to it. More info
See in Glossary that the light affects. The following are some examples of how a mask affects the sprite its covering with different Blend Modes.

Rock sprite with its original color.	Rock with a mask
Additive Light Blending	Additive Light Blending with a mask