Basics

This is a scientific law that states that the limit to something's growth/success is not determined by the quantity of all the resources, but only by the lowest one
It was originally applied to agricultural science, but can be applied to other things such as chemical reactors and even industrial managment
See the Wikipedia Page on it for more info

Applications

I like to think of it in phases of agricultural development (minus plant breeding, that kind of occurs in a seperate box) :

Think of the fist agriculture, It was barely irrigated, fertilized, carefully planted or any of that fancyness
Then you get irrigation, so the water is no longer the limiting factor
Then you get better planting and plant care practices (almanacs + plows etc), so that isn't a limiting factor
Then you get fertilizers so nutrients in the soil are no longer the limiting factor
Then you get greenhouses so climate is no longer a limiting factor
Then you get hydroponics/aeroponics so root absorbtion of nutrients (as well as oxygen levels) are no longer a problem
Then you get grow lights so that day night cycle is optimized and that limiting factor goes away
Then you get a CO2 Generator as the concentration of usable CO2 becomes the limiting factor
Then you are pretty much are only limited by plant breeding at that point (and phisical limitations in the biochemical reactions, and total energy/space available etc.)

(Granted this is a bit oversimplified, and not entirely in a sequential order like this in real life, but you get the point)