The bitwise AND (&) operator compares two numbers on a bit level and returns a number where the bits of that number are turned on if the corresponding bits of both numbers are 1. For example:

     a:   00101010   42
     b:   00001111   15       
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 a & b:   00001010   10

As you can see, the 2’s bit and the 8’s bit are the only bits that are on in both a and b, so a & b only contains those bits. Note that using the & operator can only result in a number that is less than or equal to the smaller of the two values.

So remember, for every given bit in a and b:

0 & 0 = 0
0 & 1 = 0
1 & 0 = 0
1 & 1 = 1

Therefore,

0b111 (7) & 0b1010 (10) = 0b10

which equals two.