Natural selection is nature’s way of keeping the best fit-to-live things alive, and so on continue to the next generations.
For example, if a rabbit has a limp, it is bound to be out run and eaten by a predator relatively early in its life (unless it is extremely lucky.) Because of this, it is unable to reproduce, and there genes that it contained to have its limp are unable to spread through inheritance. Natural selection goes both ways. For example, if a hawk has very god vision, it will be able to scope out its prey easily, so it will thrive. Therefore, when it produces offspring of its own, its trait of good eyesight will pass on to its offspring.
So, as enough time goes on, the bad traits will be weeded out and the good ones will remain.
