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.
They are often used with inequalities:
e.g. can be written as .
This represents the set of numbers between negative 3 and 3 (inclusive of -3 and 3).
- Khan academy online videos show more details about calculating the absolute value and a few practice problems.