I like to explain is this way. If you think about relating it to something in the real world, it works like this. Say you take a photo of yourself on the beach. Then you take a scissor and cut yourself out of the photo. In terms of Photoshop, what you have just done is "masked" out the beach.
All masks are is a way erase portions of your layers in Photoshop without making those edits permanent. You can always go back and edit a mask. But if you flat out use the eraser tool, those pixels you get rid of are gone for good. Masks, adjustment layers, and layer styles, all fall into the category of what we call "non-destructive editing".
