Its the sensor after the cat on the drivers side of the vehicle.
It measures cat converter efficiency & will throw a code if the cat or the sensor is faulty. Quite easy to replace on a difficulty rating I'd say 2 out of 5. They can sieze in the hole though & so a liberal dosage of wd40 or penetrating oil can be used followed by a good 7/8'' or 22mm box wrench if it will fit over the electrical connector, if not cut the wire.
Aeroman.