If you have ever seen an oxy-acetylene torch being used, you will have noticed that before the oxygen is turned on, the torch has a bright yellow flame. This is the fuel burning in a less than ideal amount of oxygen. The flame is relatively cool and it produces a lot of soot.
When the oxygen is turned on, tthe flame turns blue and becomes hot enough to melt steel.
You may have also seen when too much oxygen is turned on, the flame goes out with a pop.
Fuel lean is the same as oxygen rich.
In an engine, the fuel wants to burn efficiently, but not too hot that it starts to melt the pistons, or maybe even violently explode, which will also cause damage.