Pounds per Square Inch. In that context it is referring to the number of pounds per square inch over atmospheric pressure that a turbo or supercharger is compressing the air going into the engine to.
The greater the PSI, the more air is getting into the cylinders. The more air, the more oxygen. The more oxygen, the more fule you can burn. The more fuel you can burn, the more power you can make.
And WHP stands for Wheel Horse Power. That's the amount of power the car makes at the wheels (as opposed to at the crank...power at the crank...the main shaft coming out of the engine, is always greater than at the wheels because of energy that is wasted by the transmission and differential).