Wikipedia summarizes a “fitness function” as:

a particular type of objective function that is used to summarise, as a single figure of merit, how close a given design solution is to achieving the set aims.

Fitness functions are used, for example, when training machine learning models.

Training a spam filter? The fitness function would be the function that, given an individual email, returns true if it is honestly spam, or false if not. Then your machine learning model would test its accuracy against this fitness function. As the model tries new combinations of parameters, it can compare its results against this spam-or-not-spam fitness function to see if it’s getting better or worse.

What is your fitness function?

How do you know if your actions, or those of your team, are leading to better or worse results?