Masking is one of the most essential concepts in Photoshop and it is the foundation to work non-destructively with layers. Although it is a simple concept it is difficult to understand. Whenever I teach beginners to use Photoshop I always tell them to think of the mask as a black paper with a white hole in it. You can only see through the white hole. I hope this example will help you to understand masking and remember the way it works.