A layer is simply one image stacked on top of another. A layer mask is something that you apply to a given layer to control the transparency of that layer. A mask gives more precise control over very specific areas of the layer over layer opacity which controls the transparency of the entire layer immediately.
Learn more about Layer Mask:
Photoshop For Beginners: Understanding Layer Masks by WeGraphics
How to Use the Layer and Clipping Masks in Photoshop by ClippingPathService
See also:
How To Use Photoshop Layer Mask by Guru99
Free Online Layer Mask Tools: