What does it mean when a number has vertical lines on both sides of it?

Two bars on either side of a number means absolute value. Absolute value is the distance of the number from zero. That is why the absolute value of a number is never negative, it only asks how far, not in what direction. |3| = 3. |-3| = 3. It's pretty much the number that is on the inside of the bars except always the positive version. If there is a negative sign (-) on the outside, then it would work like this. -|3|= -(3) = -3. -|-3| = -3. If you still didn't understand, here is a link.