A layer mask is a feature of digital art that allows parts of a layer to be hidden without being erased for good. This allows us to fine-tune a drawing or part of a drawing without losing what has already been done. Layer masks can be adjusted over and over, hiding parts of a drawing and then being removed again to bring those parts back.
A good way to think about a layer mask is as being like a Halloween mask. Putting a mask on your face would make your real face unable to be seen. Take the mask off, and your face would be visible again.
Unlike using eraser tools, layer masks are impermanent and allow changes to be made without losing any data. We can see what a character looks like with a different hairstyle or temporarily hide parts of a background, for example. The best thing is that layer masks are easy to use and to edit!