A turbo boost gauge is an essential piece of instrumentation that indicates the amount of air entering the combustion chamber. Air indication is measured as air pressure. The greater the pressure, the greater the amount of air. This is accomplished by way of a small tube that is sealed off. The tube has a specific amount of inert gas trapped within it. Additionally, this tube runs from the gauge to the manifold, where the air flow passes. The pressure of the passing air places pressure on the tube. The tube’s internal pressure raises, and a deflection in the gauge is accomplished.
For more info, see Car Tuning: Turbo Tips