Masking lets you modify a specific bit (or bits) using a bit pattern (called the mask) and a logical bitwise operator (AND, OR, or XOR). By changing the bits in the mask you will change which bits are modified by the operator. And by changing the operator, you will change how those bits are modified.
Take for example a = 10111001 (or 185 in decimal).
Using masking, you can set the last digit of a to 0 with the AND (&) operator and a mask of 11111110 (254 in decimal):
a 10111001
mask 11111110
b 10111000
Implemented, this would look something like:
b = a & mask
You can set the first 4 digits of a to 1 with the OR (|) operator and a mask of 11110000 (240 in decimal):
a 10111001
mask 11110000
b 11111001
This would look like:
b = a | mask
You can also toggle (meaning convert all 0's to 1's and vice versa) the entirety of a with the XOR (^) operator and a mask of 11111111 (255 in decimal):
a 10111001
mask 11111111
b 01000110
This would look like:
b = a ^ mask
Similarly, implement a mask using the bitwise Not operator (~)