To NOT a value is to get the inverse of that number, or the complement; thus, any bits set to TRUE will become FALSE, and vice versa. If you applied a NOT operation to the following number:
You would get:
The resulting number is essentially the inverse of the number. Also take note that you can get the same result by applying an XOR operation to a number, using a number of the same length will all bits set to TRUE. If you applied the XOR operation to the following two numbers:
You would get the complement of the first.