MSK files are masks for the worldmap which mark the territory that is impassible for the player. Going into the ocean, for example, is prevented by these masks.
These files are in the \data folder and are named WRLDMPxx.MSK, where xx is the mask number. Each mask corresponds to one tile on the world map.
The files a bit mask, where each bit corresponds to one pixel on the world map. 1 means that the terrain is passible, 0 is impassible.
It could be said that this is just a black and white fixed-size image, one bit per pixel, without a title.
Original document at teamx.ru (in russian)