The absolute value of a difference measures the discrepancy between the two numbers inside. The 13 represents the ideal amount of biscuits and x represents the actual number of biscuits in the package. So the difference between the actual and the ideal is 4 (whether it's 4 more of 4 less than 13).
The situation described doesn't actually match the wording, because this says there'll either be 9 or 17 biscuits in the package, due to the "=". If you wanted to have "as many as," then this should be "≤", allowing for 4 or fewer in either direction.
