Fitness is the key to natural selection. We're not talking about how many reps a sea otter can burn through at the gym -- biological fitness is an organism's ability to successfully survive long enough to reproduce. Beyond that, it also reflects an organism's ability to reproduce well. It isn't enough for a tree to create a bunch of seeds. Those seeds need the ability to end up in fertile soil with enough resources to sprout and grow.
Fitness and natural selection were first explained in detail by Charles Darwin, who observed wildlife around the world, took copious notes, then sought to understand what he had seen. Natural selection is probably best explained in his words, taken from his landmark work "On the Origin of Species."
Organisms show variation of traits. "The many slight differences which appear in the offspring of the same parents may be called individual differences. No one supposes that all the individuals of the same species are cast in the same actual mould."
More organisms are born than could ever possibly be supported by the planet's resources. "Every being … must suffer destruction at some period of its life, otherwise, on the principle of geometrical increase, its numbers would quickly become so … great that no country could support the product."
Therefore, all organisms must struggle to live. "As more individuals are produced than can possibly survive, there must in every case be a struggle for existence, either one individual with another of the same species, or with the individuals of distinct species, or with the physical conditions of life."
Some traits offer advantages in the struggle. "Can we doubt … that individuals having any advantage, however slight, over others, would have the best chance of surviving and procreating?"
Organisms that have those traits are more likely to successfully reproduce and pass the traits on to the next generation. "The slightest differences may turn the nicely balanced scale in the struggle for life, and so be preserved."
Successful variations accumulate over the generations as the organisms are exposed to population pressure. "Natural Selection acts exclusively by the preservation and accumulation of variations which are beneficial under the conditions to which each creature is exposed. The ultimate result is that each creature tends to become more and more improved in relation to its conditions."
Let's delve deeper into the concept of population pressure.