Carbon monoxide (CO, not CO2) is a colorless, tasteless gas caused by incomplete combustion. It can displace oxygen in the bloodstream leading to illness or death. The incomplete combustion is typically caused by low oxygen levels, which in turn is caused by insufficient ventilation.
Some devices, notably the Mr. Heater Buddy, have low-oxygen shutoff mechanisms to minimize CO exposure. O2 interlocks can cause issues with operation at higher elevations (8000'+).
To avoid CO poisoning: