What is the meaning of Ones'_complement?

The number obtained by subtracting a given n-digit binary number from ${\displaystyle 2^{n}-1}$ (which yields the same result as the logical complement).

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

Source: wiktionary.org