i read many time 12 psi and 13 psi and i dont know what it mean so can anyone help me ?

pounds per square inch. So you'd have 12 or 13 pounds of pressure for every square inch of space.

As was stated, PSI is short for Pounds per Sqaure Inch. At sea level, the air pressure of the outside air is 14.7 PSI. When someone says they are running a car with 12 psi of boost, they mean they have forced 12 MORE psi into the motor with a turbo or supercharger...that's 12psi above the natural ambient air pressure. It's typically referred to as just 12 psi though, as most people don't refer to the ambient air pressure at all.