Good thing I didnt sleep through Thermodynamics 101 during freshman engineering
In a combustion engine, it is imperative that the air to fuel ratio be monitored and controlled, the latter which is done through manifold fuel injection systems in modern cars. A close loop feedback system, where the O2 sensor provides the feedback (coupled with some other sources on board the vehicle). So basically the sensor tells your car's on board computer (PCM) how to adjust this ratio.
If your sensors bad, please get it changed, i) because your car might fail an emissions test if required OR the engine might misfire during heavier loads. Your check engine light should be on if the sensors kaput.