There are some other uses of the word 'mask', which arise by extension. That means using 'mask' as a metaphor. In film a mask is a screen which cuts out part of the image. In social theory, a mask is the behaviour a person takes on in his role or job. Thus policemen, head teachers, army commanders, prime ministers act as they are expected to act (most of the time). The inference is that behind the mask is the 'real' person. Their role is a mask. As a verb, 'to mask' means to cover or hide.