GRASS Raster Mask

Q:: I do not have the concept of mask. Please anybody help me in understanding mask. What is this?

A: A mask allows you to block arbitrary areas of all rasters processed while the mask is set. That is, for cells where the mask is set, those cells will be treated as if they were NULL. It was more important when GRASS didn't have a true concept of NULL value (GRASS 4.x), however, masks still can be useful even in GRASS 5.x

Masking can be even used in ps.map script, so a raster mask can applied to vector features that one wants to be masked, it is done by the masked y|n switch.