Any unsigned integer 'n' will have the following last k digits: One followed by (k-1) zeroes: 100...0
Note that k can be 1 in which case there are no zeroes.
(n - 1) will end in this format: Zero followed by (k-1) 1's: 011...1
n & (n-1) will therefore end in 'k' zeroes: 100...0 & 011...1 = 000...0
Hence n & (n - 1) will eliminate the rightmost '1'. Each iteration of this will basically remove the rightmost '1' digit and hence you can count the number of 1's.
