Darwin’s theory of evolution explains that evolution occurs from natural selection.
Individuals from each species each have a certain set of physical traits.
One individual will have a set of traits that is more suited for their environment and will survive (where as the less suited ones will not) and very very slowly overtime that particular species will evolve into another species, or evolve.
Darwin’s point was that basically, every species that is alive today was not “created” individually, but evolved from simple life forms to more complex and diverse life forms.
Hope this helped! (: