It means absolute value. Basically, if the number is positive or equals zero, then putting it between | and | does not change its value, whereas if the number if negative, the sign is removed :

\(\displaystyle |3| = 3\)

\(\displaystyle |7| = 7\)

\(\displaystyle |0| = 0\)

\(\displaystyle |-2| = 2\)

\(\displaystyle |-271| = 271\)

Careful when analyzing functions using these symbols with an \(\displaystyle x\) term in it : you need to consider two different functions : when the expression between the symbol is negative or positive.