That's called the absolute value function, and it's defined by
|x| = \begin{cases} x, &\text{if }x \ge 0, \\-x, &\text{if }x < 0.\end{cases}
Basically, you only want the magnitude of the number, not its sign.
For example,
|-1| = 1,
|-2| = 2,
|1| = 1,
|0| = 0.
If x = |y| , then...