The vertical bars signify "absolute value" of whatever is between them. Recall that the absolute value of something means its distance from 0, and is always given as a positive number or zero, but never negative. So for the function f(x) = |x-5| some of the values would be:

f(-10) = |-10-5| = |-15| = 15

f(0) = |0-5| = |-5| = 5

f(5) = |5-5| = |0| = 0

f(10) = |10-5| = |5| = 5

f(15) = |15-5| = |10| = 10