Carbon dioxide (not to be confused by carbon monoxide, the extremely deadly gas that we also monitor for) is produced by almost all living things, especially us.
Long exposure to small amounts can affect your concentration, energy levels and can cause headaches. But besides that, a buildup of CO2 indoors means that you have not gotten fresh air in your house for a while, which is an indicator of other harmful effects, such as the buildup of fumes slowly given off by your furniture, paint and other household compounds, or just an indicator that it's time to open some windows. In that case, we will be able to let you know.
Larger levels of carbon dioxide can buildup in areas that store carbonated beverages, or when there is some form of combustion occuring. Levels of around 3' will cause you to start breathing at twice your normal rate, and cause dizziness, increased heart rate and blood pressure, while above 10' can cause suffocation. But Birdi will warn you and the people that need to know long before that happens.