You have a exhaust oxygen sensor fitted to your car, somewhere in the exhaust system. This senses the amount of oxygen in the exhaust and sends a signal to the ECU (engine management system), which then regulates the amount of fuel being delivered to the engine, thereby achieving the correct exhaust reading.

There may be a number of things that have caused this problem. The catalytic convertor may have failed, there could be a fault in the ECU or with the exhaust sensor, or it may be something as simple as a blocked air filter.