How does fitness relate to natural selection?

Fitness is a measure of the ability to survive and produce more offspring relative to other members of the population a given environment. Natural Selection is is a mechanism by which individuals that have inherited beneficial adaptations produce more offspring on average than do other individuals.

So how does these two things relate? But the offspring must survive to contribute to the following generation, so fitness as a generational value. An individual's fitness became defined as to how well that individual contributed its genes to the next generation. The bottom line is still how many successful offspring an individual has.