Take a 64 bit number and treat it like a bit field where each bit represents inclusion/exclusion of an element in a set S of size 64. Thus every number with M 1s represents a (distinct) M-sized subset of S

Every bit in a bitfield represents a "thing". The bitfield represents a collection of distinct things. If bit "i" is 1, then the corresponding thing "i" is in the collection.

Iterates through all possible N-combinations of a set that has M elements?