A well pump pressure is controlled by the pressure switch normally located close to the pressure tank.
The pressure setting is usually a matter of the users choice, but other factors may also be involved such as the pressure the pump is able to supply.
Many systems are calibrated to turn on the pump when the system pressure is reduced to 30 psi while water is being used in the home, and will shut off the pump at 50 psi. Most people prefer higher pressure so 40/60 psi has become common.
If the pump can supply more water than is being used, the excess water from the pump is then stored in the pressure tank. Once the tank pressure reaches the shut off pressure, the pressure switch will shut the pump off, but when water is again or continues to be utilized, the pressure will soon lower to the cut-in or turn-on pressure so the pump will run again and the process is repeated over and over.
Because pump cycles will damage a pump more than running it continuously, review the video Valveman attached to post #2 as his CSV device will extend the life of a pump by reducing cycling, and will provide more consistent pressure to your home faucets while water is being used.
If you are only using 3 gpm while showering, with a CSV, the pump will then only supply 3 gpm. If someone flushes a toilet and flows a further 2 gpm, the CSV will then allow the pump to flow 5 gpm at the same pressure so you are unlikely to notice while your shower is running.