In simple terms fuel won't burn on its own (it needs air, or more specifically it needs oxygen), it won't burn in liquid form. As a vapour though it will burn. The fuel is mixed with air, but they are not mixed equally there is considerably more air than fuel.
The fuel and air is mixed to give an efficient ratio,
The ideal fuel:air ratio is known as the stoichiometric ratio
if too high a fuel:air ratio is used the mixture is said to be 'RICH'
if too low a fuel:air ratio is used the mixture is said to be 'LEAN'
The fuel:air ratio is the ratio of the masses of fuel and air.
The fuel/air mixture is mixed in a metering unit known as a carburettor in a normally aspirated engine or a servo injector unit in a fuel injected engine. The fuel:air ratio is controlled by a mixture control lever in the cockpit. The mixture is metered based upon the volume of gas drawn into the metering unit, which means that as the aircraft climbs and the air thins the fuel mixture becomes too 'RICH'.
