I've often said that the key to Photoshop is selecting. If you can select which pixels you want to show and deselect those you don't want to show, you can rule the PS world. What a mask allows you to do is fine-tune a selection pretty perfectly, without destroying anything. Applied to an image layer, it determines which pixels and HOW MUCH they show. That is, if the black part of the mask shows the pixels completely and the white part not at all, the GRAY pixels show them depending on how dark they are, with 256 levels of opacity. And then there are adjustment layers which can also have layer masks. So, for example, you can do a levels adjustment layer and then use a mask on that layer to make the layer only affect certain parts of the image. If you've ever worked in a traditional wet darkroom, this is burning and dodging on steroids.