Sentence:  That is, the lower nibble (the 4 bits 1010 = A) has been <WSD>masked</WSD> to zero. This is because ANDing anything with a zero produces a zero, while ANDing any bit with a 1 leaves the bit unchanged... 
Meaning of the word marked with <WSD> tags:  To set or unset (certain bits, or binary digits, within a value) by means of a bitmask.