Pump Head is the linear vertical measurement of the maximum height a specific pump can deliver a liquid to the pump outlet.
A pump can only deliver a liquid to a maximum height because the weight of the liquid above the pump mechanism becomes greater than the forces which are attempting to turn the pump or move the piston or diaphragm.
Pumps are able to deliver more volume of liquid at a lower outlet height or "head". Pump head measurements are given in volume flows at various head heights. The additional work required to push the heavy column of water up with each cycle of the pump mechanism requires proportionally more energy to move the same amount of water to greater heights.
Appropriately sizing a pump for head volume and pump lift is important for the life expectancy of the equipment and to reduce the amount of power the pump will consume over its lifetime.
A pump installation at a fuel dock draws fuel from an underground tank and delivers it via a dock mounted pump. Vessels docking there vary in size and fuel filler height.
One ship has a filler height of two meters and another has a filler height of four meters. The fuel pump has a maximum effective head (the point where flow rates are still acceptable for the application) of five meters.
Because the pump is nearing it's effective head when filling the taller ship it will provide less volume per minute than when filling the ship at two meters. If the pump had a maximum head of twenty meters there would be less difference in fill time between the short and tall vessels.