The pump head is another way of expressing energy. Like common misconceptions, it is not another way of expressing pressure or the direct measurement of the height to which a pump can deliver the fluid. In fact,
Pump Head = Static head + Velocity head + Friction head
Static head or pressure head is the measure of the energy associated with the pressurized fluid. When somebody tells a pressure head of 10 meters for a fluid, it means the pressure there is equal to what a tank with 10 meters of the same fluid experience at the bottom.
Velocity head or dynamic head or kinetic head is a way to express the kinetic energy associated with the fluid flow. Suppose we have a pump delivering at 100 liters per minute to a height of 10 meters. What if we make it deliver to 15 meters? The flow rate reduces i.e., the velocity of flow reduces. What we are doing is trading off velocity head for pressure head. You can consider velocity head in a pump as the kinetic energy that is lost at delivery or the energy which could have been converted to pressure head to pump to a higher height.
When fluid flows through pipelines, flow obstructions (like bends, elbows, debris, viscous forces, and friction between fluid and pipeline) causes pressure losses. These losses when converted to units of length forms the friction head.
The sum of all three forms the total head value of the pump.
Most pump manufacturers specify pumps on head and not pressure. They name the pump head as the total head value of the pump. But most pump users name the pump head as the static head value of the pump.
Suppose a pump is specified for a head of 15 meters at 100 l/m flow rate. Here, 15 meters is the total head of the pump (measure of the total work done by the pump). Assume there’s 2 meters friction head loss in the pipeline and the head associated with exit velocity is 3 meters. So, 15-2-3 meters=10 meters is the static head. This 10 meters is the maximum height to which the pump can deliver at 100 liters per minute.
If you go below 10 meters, the flow will increase. If you are above 10 meters, flow reduces until it reaches the shut-off head of the pump (shut-off head is the maximum upper limit of pressure the pump can produce). When it reaches shut-off, the flow ceases.
So, pump head gives an idea about the height to which the fluid can deliver. (Normally, velocity head is very low and can be neglected). Pump head is approx. 80–85 % of shut-off head and an idea about shut-off can also be obtained by defining pump head.