PSI stands for pounds per square inch. It is a unit of pressure [force exerted on a surface]. PSI is commonly used to measure the pressure of gases or liquids in various systems. For example, a car tire may have a recommended PSI of 30, meaning the air pressure inside the tire should be 30 pounds per square inch.
Very Simple Definition:
PSI is a way to measure how hard something pushes on something else. It is usually used to talk about how much air is in a car tire or how hard the water is coming out of a hose. For example, if the PSI in a car tire is too low, it may feel bumpy and unsafe to drive.