A clipping mask is a layer whose content and transparency is used to control what is visible in the masked layer(s), in a non-destructive way.

Clipping masks are a great way to control the shape/content of image layer(s) without altering the content of the base layers (non-destructive). This allows either the mask or the base layer to be easily, rapidly, and reversibly altered, independently -- to achieve the desired effect.

Public domain example of clipping-mask effect:

         example of clipping mask in action