The PSI conveys the amount of water pressure that a piece of clothing can endure from the pressure of the wearer to the external conditions of the weather. For a jacket to be classed as 100% waterproof, the British Standard of 3 PSI must be applicable, meaning it can survive 1500mm of water pressure.
Of course, pressure is not as clear-cut as the individual rainstorm, as by moving the jacket you are applying pressure to the fabric. Normal activities such as kneeling and walking will generate a massive 8000m of pressure, so it is always recommended to go for a higher PSI rating if you are solely basing your decision of jacket on this.