Not a chemist but what the heck, I'll play one for now.
Most fuels are composed of hydrogen and carbon. In order to fully release all the energy stored in the fuel, there is an ideal amount of oxygen required for the chemical reaction. Too little air and it doesn't fully react. Too much, seems hard to occur but it certainly does inside of an internal combustion engine (due to the small window of time for it to do "useful" work and the need for the flame to go from combustible molecule to another one). Ultimately you get carbon dioxide and water--and heat which is what is usually we are after--and to do that there is a proper amount of oxygen (from the ambient air) required.
Now I realize fuels can be other than hydrogen and carbon based, and often there is stuff in the fuel which also requires combusting (which may or may not help with releasing heat) but most stuff that we like to burn is largely hydrogen&carbon (oil, gas, wood, coal).