Pressure is measured in pounds per square inch or psi. A pools ideal pressure depends on the size of the pool, type of filtering system installed and filter media used. Once the ideal pressure is established, the gauge should be monitored for variations, which show problems with the system.
When water is circulating properly, and filter tanks are clean, the pressure gauge will read close to the ideal pressure. On the other hand, when the pressure is high, there is a problem after the water leaves the pump. Most likely, high pressure and low flow indicates a dirty filter. Backwashing may be necessary to lower the pressure and increase flow. And if the pressure is consistently high, even after cleaning the filter and backwashing, it could be time to replace the media or cartridge or to check for filter obstruction.
Low pressure and low flow indicates a problem before the water enters the pump, like a clogged basket, a blocked drain cover, low water level in the pool, an air leak, a clogged pump impeller or a malfunctioning pump motor. The easiest solutions to try first are adding water to the pool and cleaning the baskets. If the pressure does not go back to normal, turn off the system and contact a service provider.