Usually you would be talking about bit-masking and essentially it means determining if the value of something will have an effect on the next layer of the operation. (If you use Photoshop, think similar to the masking there).
If thats a bit mumbo-jumbo try this example;
Original info = 1011 1100
Bit mask = 1110 0000
Info sent = 101
Think of the bit mask as a gate, where if it’s a 1 then information is let through, but if its a 0 then no information is let through.
Note where a bit mask is 0 then no information is over-written, its not that it just writes a zero to the destination (that would be a bit-wise and) hence why the info sent is 101 not 1010 0000