Head refers to how high (meters or feet) a pump can move water or another fluid up through a building. Head is a crucial part of calculating what types of pumps you will need in a system, and how they should be arranged.
Let’s say you have a single pump in the basement of a 6-story hospital building, but that pump’s head is not capable of pumping the water up to that 6th floor.
You might think you can just increase the waterflow rate (pump water through faster), but this would actually decrease the head because the increased flow causes more friction in the system.
One way to increase the head of the system is to add another pump in series in that system. These two pumps could maintain the same flow rate while combining their separate head measurements to move the water up through the building.