What is fitness? Why is it relative?

In the course of natural selection, amazing and diverse biological adaptations (adaptations) of organisms to environmental conditions in which the life of a population proceeds are generated. For example, general adaptations, which include fitness for swimming of organisms living in the aquatic environment, or fitness of limbs of vertebrate animals to the terrestrial environment, and private adaptations: fitness for running in a horse, antelope, ostrich, digging in moles, mole rats or climbing trees (monkeys, woodpeckers, pikas, etc.). Examples of adaptation are masking color, mimicry (a peaceful appearance imitation of the appearance of an animal that is well protected from predators’ attacks), complex behavioral instincts, etc. It should be remembered that any adaptation is relative. A species well adapted to these conditions may be on the verge of extinction if the conditions have changed or a new predator or competitor has appeared in the environment. It is known, for example, that fish that are well protected from predators by spikes and spines are more likely to fall into the nets of the fisherman, in which they become entangled and held just because of solid outgrowths of the body. No wonder one of the principles (evolutionary doctrine) in a joking manner is: “The fittest survive, but they are fittest only as long as they survive.”
So, opportunities for evolutionary changes in the population are always present. For the time being, they are manifested only in the variability of organisms. As soon as selection begins to act, the population responds with adaptive changes.