Evolutionary biologists use the word fitness to describe how good a particular genotype is at leaving offspring in the next generation relative to other genotypes. So if brown beetles consistently leave more offspring than green beetles because of their color, you'd say that the brown beetles had a higher fitness.

Evolutionary Fitness is how well a species is able to reproduce in its environment. If they're no longer reproducing then they are no longer evolutionarily fit.

Importantly, all those factors depend on the specific environment in a species' habitat. This fact is strongly related to the concept of niches: in each niche a different species has potentially the largest fitness and outcompetes less adapted ones.

Most often, measures of fitness involve these key areas: Aerobic fitness, which involves how well the heart uses oxygen. Muscle strength and endurance, which involve how hard and long muscles can work. Flexibility, which is how able joints are to move through their full range of motion.