Given a binary number x, the number NOT(x) is called the 1's complement of x. The NOT operator when applied on a bit reverses the 'truth' of the bit i.e.
NOT(1)=0          and   Not(0)=1
We extend the NOT operator to a multi-bit value, so that Not(x) is the value with all its bits reversed.
So, the 1's complement of:
1010 1010
would be:
0101 0101
the binary number inverted.