The absolute value of a number refers to its "distance" from a standard zero point. This concept is most simply explained in terms of points along an x-axis, where 0 is located at the origin, all points to the right of the origin are positive, and all points to the left of the origin are negative.
The value 5 intervals to the right of the origin (or zero) is +5. The value 5 intervals to the left of the origin is -5. However, the distance of each of these two values from the origin is the same: 5. "5" is the absolute value of both +5 and -5.
Mathematically, "absolute value" has a more formal definition. Say x is a real number. Then the absolute value of x is defined as follows:
For x ≥ 0, |x| = x;
For x < 0, |x| = -x
where the vertical bars around "x" denote "absolute value of."