A fitness function is a single parameter that is used to define how effective some solution is at resolving some constraint. For example, this might be how efficient a car is at converting fuel into motion, or how effective a given creature is at intercepting and processing resources within a given ecosystem, or it may be how competitive a country is within the global economy. All of these could be defined according to a single metric and compared across the different elements in the system, this single metric we would call the fitness function.