A pressure switch is a device that is configured to sense a change in a certain pressure on something and then react in a certain manner. Pressure switches are used on many devices such as electric and gas heaters, pumps, small air compressors, electric strip heaters, food processing equipment, and of course on many hot tub heaters.
When it comes to your hot tub, the pressure switch (also often referred to as a flow switch), makes sure that water is flowing while your heater is on so that in the event water is not flowing your heater will not over heat, and burn itself up. It is a safety feature that keeps the heater from turning on when there is not water flowing. The switch senses water pressure and if the pressure is where it should be the switch completes the circuit of electricity to the heater.
If you are having trouble with your spa heating or heating insufficiently, the problem may not be with your heater. It could be a pressure switch issue. If your pressure switch is not working properly it could cause the water not to heat, but always make sure you have water flow because the spa not heating could also just be the pressure switch doing its job.