The way your engine works is by mixing air and fuel which causes and explosion and produces power... Gasoline as a liquid doesn't burn... It needs a catalyst, like oxygen, obviously found in the air we and bike breathes to make the boom...
I don't really know what the "perfect" air/fuel mixture is for the ninja, but you don't really have to....
So, building off of what D said, Rich = too much fuel for the amount of air. If an engine is very rich, you could foul plugs (carbon build up on the plugs that makes them not work), and the engine will be down on power, and it will run cool. It will not be giving it's all, but it is safer.
Lean = too little fuel for the amount of air. If the engine is very lean, you burn valves, produce a lot of horsepower, run hot, and damage a whole lot of stuff....
It's about getting the perfect ratio. Atmoshperic conditions also play into, since they affect how heavy/thick the air is. Altitude does as well..
Without getting too deep into the rest, ask another question so I don't bore you...
