Data Mask pin as name suggests masks the data from getting writen into the memory.
Suppose you want to mask 3rd bit in 8 bit system. Then you will set mask pins
00000100 (left most LSB). So, your information at 3rd bit is not distrubed during writing. This masked bit can be some sort of flag which is set by some previous operation which user do not want to change during next write operation into the memory.