In short, a value can hold a negative value if it is a signed value.
This is done by inverting all the bits. All 0's become 1's and 1's become 0's. Then the resulting value is
incremented by 1.
example:
12 = 00001100
invert = 11110011
-12 = 11110100 (notice that if this was an unsigned value this would be 244; because 256-12=244)