The oxygen sensor "tells" the computer what the oxygen content is by providing a voltage on it's signal wire between 0 and 1 volt. 450 millivolts (.45 volts) means that the fuel/air mixture is correct. Higher values means the mix is rich (has too much gas), and lower voltages means the mix is lean. By adding voltage to the sensor's output, we can compensate for the additional oxygen in the exhaust.