The primary source for CO2 indoors is human exhalation.  This compound is a byproduct of human metabolic activity and might be more likely to build up in areas with a lot of people or poor ventilation.  Other sources include those resulting in combustion byproducts: car exhaust, fossil fuel power plants, leaking furnaces, cigarette smoke, etc.  CO2 can be considered one of the primary products of combustion; complete (or ideal) combustion results in CO2 and H2O and energy.  CO2 is a greenhouse gas and a major contributor to climate change. 

Learn more about the carbon cycle