Kart and motorcycle engines use a mixture of oxygen and petrol. The engine gets oxygen from air pulled in thru the venturi and the petrol through the jets. To get the best performance you need the correct Air to Fuel Ratio (AFR). When the Air Density goes up or down we need to bring the AFR back by leaning or riching the Jet.
Relative Air Density helps you understand how much oxygen is in the air. This changes based on the Relative Humidty, Air Pressure and Temperature. When the RAD is lower you need to use a smaller jet and when it increases you need to use a bigger jet.