Question

What does | mean in math for example; |15|

Answer 1

It means to find the absolute value of a number.

E.g. |-7| = 7
E.g. |59| = 59

The absolute value of a number refers to the distance of a number from the origin of a number line.

All negative numbers become positive!!!

Answer 2

Those markings represent "absolute value"

It basically means to remove any negative sign in front of a number, and to think of all numbers as positive.