Mother nature plays an important role in how high we can push water. Water is heavy; about 8.3 lbs per gallon. The old saying, "what goes up, must come down" tends to want to bring the water back down to its source. The mechanical energy of the impeller transmits its force against the water coming in contact with it. This force can be measured in psi at the pump discharge. As the pump discharge head increases in height, the pump capacity (gpm) decreases and the available pressure at the end of the discharge hose (if the flow is stopped or a sprinkler/nozzle is used) will also decrease. At maximum head, the capacity (gpm) will drop to zero and there will be no pressure available at the end of the hose to run a sprinkler or nozzle. If we measured the pressure at the bottom of the discharge hose, we would read maximum head pressure which would be the result of the pump supporting the water weight.
The performance curves show the relationship between discharge capacity and total head.