Carbon monoxide is a gas produced on the basis of incomplete burning of carbon-rich combustible material. This is because only half of the oxygen bonds to carbon, forming carbon monoxide.
When combustion occurs, it can be complete or incomplete. If complete, carbon dioxide formation occurs, if incomplete, carbon monoxide formation occurs.
Example of incomplete combustion of butane:
2 C4H10 + 9 O2 ---> 8 CO + 10 H2O
Butane reacts with oxygen gas (that is in the air) and forms carbon monoxide and water.