Absolute value

Straight lines on both sides of an expression or number are called absolute value signs.

The absolute value of a number is its value when the sign is removed, i.e. the distance of a number to zero.

eg. $|-3| = 3$ e.g. $|5-12| = |-7| = 7$

They are often used with inequalities:

e.g. $-3 \leq x \leq 3$ can be written as $|x| \leq 3$ .

This represents the set of numbers between negative 3 and 3 (inclusive of -3 and 3).

Further information

Khan academy online videos show more details about calculating the absolute value and a few practice problems.