Think about your car. Your car has an engine that requires gasoline to operate. To get from point A to point B, the engine burns some gasoline. The longer the distance, the more gasoline is required. You purchase gasoline at a gas station and pay money for it. However, how do you estimate how much a certain trip will cost you in terms of money? First, you estimate total consumption of gasoline. For this, you take approximate distance of the trip (e.g. in kilometers) and multiply it on average consumption by your car (say 9 liters per 100 km). Further, you multiply total consumption on the price of gasoline (e.g. 1.55 Euro per liter). Thus, you arrive at an approximate cost of the trip.
Ether...