Agree with David White's reply, here is some additional background which may be helpful.
The job of the pump is to turn shaft horsepower into water movement. Horsepower at the pump shaft is the product of a flow (RPM) and an effort (torque) variable. Horsepower at the water outlet is the product of a flow (pounds per hour) and an effort (pounds per square inch) variable.
How do these vary with the speed with which the pump is running? Let's look at the limiting cases: no hose connected to the pump, and hose outlet blocked.
The no hose case represents the condition where the pump output is not connected to a load of any kind- the water runs free. In this case the pump is producing no useful work except that required to pull water through it: lots of flow, almost zero pressure.
The blocked outlet case represents the condition where the flow from the pump is not allowed to drive a load of any kind- the water goes nowhere. In this case the pump is producing no useful work except that required to pressurize the blocked outlet pipe: No flow, lots of pressure.
At both of these extremes, then, the delivered power of the pump is zero.
Remembering that power = effort times flow, we then ask when is this product a maximum? To maximize the product of two numbers requires that we simultaneously maximize the magnitude of both numbers, which means that the best match of the power in to the power out occurs when the pressure and the flow rate both go through their maxima which occurs somewhere between the two minima that bound the power curve. This is the "design point" of the pump.
If we then reduce the load by opening up the nozzle, we get more flow- but at lower pressure. If we increase the load by closing down the nozzle, we get less flow- but at higher pressure.
From here, the analysis can be expanded by going into the details of how the pump works and how the driving engine's power output varies with speed and load, but the general principles outlined above hold regardless of the engine's and the pump's interior workings.