The absolute value of a number is the number's distance from zero on a number line.
Since we never talk about distance as being negative, the absolute value of a number is always positive. Think of it as the number of jumps it would take to get to zero from a number.
We use bars (vertical lines) on either side of a number absolute to mean absolute value.
|-4| = the absolute value of -4 = 4
Pretty simple. If the number is negative, make it positive. If the number is already positive, leave it alone.