The vehicle computer monitors the air/fuel ratio of the exhaust. It uses Oxygen (O2) Sensors to maintain the proper air/fuel ratio, i.e. 14.7:1, through the fuel system.
The O2 Sensor produces a voltage reading that is used by the PCM (Powertrain Control Module). If the PCM reads that the actual air/fuel ratio is well within the required range so that the PCM cannot make adjustments, it sets the DTC code P2195 OBDII.