It is referring to reading a too rich condition on the front O2 sensor. So most likely this is either a bad O2 sensor or a bad MAF causing it to run rich.
Some other causes which may be less likely but still possible:
+ Vacuum leak on FPR (this is the little hose under the plastic engine cover running from the intake to the FPR)
+ Bad temp sensor (this would cause the engine to always think it is cold and therefore run in the equivalent of "choke" mode).