Natural selection is an idea that Charles Darwin came up with.
It attempts to describe how animals and plants evolved by using nature as the determiner of who survives. For example, if there is a fast cheetah population and a slow cheetah population in Africa, the slow cheetah population will eventually die out due to them not being able to catch prey to survive. The fast cheetah had the desirable traits necessary for survival in this location. Only the strong survive is helpful to remember when dealing with natural selection.