Natural Selection is a theory of evolution based on the premise that organisms which have genetic traits that make it easier for them to survive in an environment and more attractive to the opposite sex are more likely to pass on their gene pool to future generations. This makes future generations acquire their genetic characteristics and alters the genetic pool of the species.
Natural selection works in two ways, the first being that the genetic traits make it easier for some organisms of a species to survive in an environment. Examples of this could be genetic traits that make the color of some insects such that they blend in easily with the surrounding and this saves them from predators. Insects which do not have this trait are easily hunted which reduces the number of organisms with alternate genetic traits.
The second is that the genetic traits of some organisms make it easier for them to attract the opposite sex and mate. They are in this way able to pass on the genetic traits which make them look more attractive.
Usually the gene pool that results in genetic traits making organisms of a species more attractive is also the one which increase their ability to survive in the environment. For example, the most brightly colored birds are also the ones which are the strongest and can easily find food. This makes the opposite sex attracted to them and they have a higher probability of passing on their gene pool to future generations.
Natural selection in this way changes the genetic traits of a species over time by selecting the gene pool of the organisms which have features more suitable for survival in an environment.