It means that the, while monitoring the O2 sensor, the PCM "saw" a continuous "lean" reading. Normally the PCM adjusts the fuel injector pulse width to eliminate the lean reading, but it sets this code when it cannot "enrichen" the mixture enough to compensate for the "lean" reading. There are several possible causes:
1) The O2 sensor could be bad, sending a false "lean" reading to the PCM.
2) The wiring between the O2 sensor and the PCM could be bad, causing the PCM to be unable to see the O2 sensor.
3) Something in fuel delivery (low fuel pressure for example) is causing the engine to run lean.
4) A vacuum leak, or some other fault in the air intake, is allowing unmetered air into the system, causing the engine to run lean.
And, another cause for a lean code like that during the KOER test, is performing the test on a cold engine. The O2 sensor requires a certain amount of run time A couple of minutes to reach operating temperature. If it isn't at it's operating temperature during the KOER test, it will set a lean code simply because the O2 sensor was too cold to give a reading.
