An internal combustion engine is running Lean when it has less fuel in the fuel/air mixture than it would have at the point of greatest stoichiometric efficiency. The penalties include premature detonation, which increases heat, reduces efficiency, and will damage your engine.
Running lean will also damage your spark plugs. Overheating will cause the electrode to be glazed, and the core "nose" (the ceramic piece around the core) will remain close to its original appearance. Detonation can cause the electrode and core to burn away, with the core nose having a burnt appearance. In more severe cases, it can even cause the core nose to chip or shatter. Any of these conditions will cause suboptimal sparking.
The antonym of lean (in this context) is rich.