Air is a general term used to describe the collection of gasses in earth's atmosphere. Typical automotive engines which run on gasoline or diesel to power internal combustion need the oxygen (O2) from air to make combustion possible.
Composition of Air
When free of humidity/moisture, air contains roughly 78% nitrogen, 21% oxygen, .93% argon, .041% carbon dioxide (CO2), and trace amounts of other gases. The amount of oxygen can be artificially increased to improve power output.
Air's Role in Combustion
Combustion, a form of rapid oxidation, releases energy mostly in the form of heat as the oxygen is chemically bonded to elements in the fuel. The heat causes the remaining air to expand, pushing down on the piston. After this, the piston pushes out the post-combustion gasses. If the vehicle is equipped with them, the O2 Sensors will try to detect remaining oxygen, which is either a sign of incomplete combustion or a poor air/fuel ratio.