A bitmask is an integer where each bit represents whether an element is included. With elements labeled to , bit being means element is in the subset.
Example with : mask in decimal in binary. Bits , , and are set, so this represents . The empty set is . The full set is (all bits set). This mapping is one-to-one: every subset has exactly one integer in , and vice versa. This lets us use integers as array indices for DP (dynamic programming).