What is the meaning of Twos_complement?

The number obtained by complementing every bit of a given number and adding one. A number and its complement add to 2, where n is the word size of the machine.

The convention by which bit patterns with high bit 0 represent positive numbers from 0 to $2^{n-1}-1$ directly, while bit patterns with high bit 1 represent negative numbers from -1 to $-2^{n-1}$ , n being the word size of the machine, and the numeric complement of a number is its two's complement.