Sometimes it can be helpful to represent it in binary to get a better visual representation of the data.
As an example -1 represented in 16 bit signed is 1111 1111 1111 1111. The hex representation condenses this to FF FF.
Unmasked this would be transmitted as shown above but it can be masked which essentially overwrites the bit value to 0. 65 72 in binary is 0110 0101 0111 0010, so any bits which are 0 in the mask value will be overwritten as such and any bits which are 1 in the mask will be unmodified from the original value.
Basically this is just a clever way to transmit any value we like without needing to use an existing channel value.