What is excess air?
There is a theoretical amount of fresh air that when mixed with a fixed amount of fuel, and burnt will result in perfect combustion. In this situation all of the fuel will have been properly burnt and all of the oxygen in the air will have been consumed. In this circumstance there will be no excess air and combustion efficiency will be maximised.
In the real world, perfect combustion is not possible. The theoretical amount of fresh air would provide insufficient oxygen for complete combustion and some of the carbon in the fuel would be converted into carbon monoxide rather than carbon dioxide. A lack of air can lead to dangerous levels of carbon monoxide being formed and smoke being produced.
Therefore it is usual to adjust the combustion process so that a level of excess air is present to give margin safety. This level is set to account for any likely process variable, e.g. The variability of the fuel supply, changes in atmospheric pressure, changes in wind direction etc.