Even though we refer to natural selection as a "process," it's better described as "an observable phenomena," a "mechanism," or a "concept." This concept states that the organisms with the best fit traits for an environment have a greater chance of survival, tend to live longer, and have more opportunities to reproduce. These organisms are "naturally selected" for survival. If you've ever heard the term "survival of the fittest," that is literally describing natural selection.
Watch the videos below to understand how the process of natural selection can cause the population of a species to change over time.
Environmental pressures increase or decrease the advantages of a trait which in turn increases or decreases this trait in a population. Over LONG periods of time, this trait changes a population of an organism in a given area. Evolution has occurred.