Two's complement

For representing +ve and -ve numbers.

The most significant bit (leftmost) bit:
0 => 0 or positive
1 => negative

e.g. 01110000 => +ve
e.g. 11110000 => -ve

If the left most bit = 0,

01110000 = 2^6+2^5+2^4 = 64+32+16 = 112

If the left most bit = 1, invert every remaining bit and add 1.

11110000 => 0001111 + 1 => 0010000 => -16

Reference:

Two's complement (Wikipedia)