A mask, in computer science jargon, is a bit string used with the AND bitwise operation as a data filtering technique.

Say you have a byte string of length 128, representing an entry in a database or something. This string contains different fields - perhaps the first 8 bytes are a person's first name, then 8 for the person's last name, one for an initial, 9 for a Social Security number, and 38 for a comment. If you simply want to analyze the last names of the person, for sorting or reporting or whatever, you can separate this information with a mask. This specific mask would contain 1's for bytes 9 to 16 and 0's everywhere else, effectively removing all other data from the string. Of course, you wouldn't want to copy this string over the initial string, or you'd lose the rest of your information!