What does two lines next to a number mean?

Answer 1

Those lines are an algebraic operator that marks an absolute value. If the value surrounded by the lines is negative, it becomes positive. If it's positive, it remains so. For example:

|-9| = 9

|9| = 9

|5 - 7| = 2

|7 - 5| = 2

If f(x) = |x2 - 5|, then f(x) describes a parabolic curve for which the values in the range -5 ≤ x ≤ 5 are positive rather than the expected negative. It would look vaguely similar to a calligraphic letter "w", with the bottom two points being on the x axis.