The notation for taking the absolute value is two vertical lines on either side of the expression being evaluated. Taking the absolute value of a number is the same as finding the distance from zero on the number-line of the number.
In practice, the absolute value function can be used to calculate the absolute value of a number.
The distance between two points, in one dimension, is given by the absolute value of the difference between the two values.
The distance between two points, in two dimensions, is given by solving pythagorean's theorem for the length the hypotenuse of the right triangle formed by the two points.