This will set the value of variable to NOT number. The NOT operation simply inverts all of the bits.
In 'human' terms, this usually means:
A positive number becomes negative, less one; i.e. 4 becomes -5.
A negative number becomes negative, less one; i.e. -5 becomes 4.
Example
A = NOT 5
not(A, 5)
Notes
The difference between this and Exclusive OR is that the eor() command takes a mask of bits to invert. This simply inverts all of the bits.