It means that's we can have the same kind of species but different phenotype. For example monarch butterflies. One is brown. the other black. Let's say they are located on the beach , sitting on the sand. The black one would be less likely to survive since it's physical characteristics are giving it a disadvantage in this case it's color. Therefor the brown one would survive and thrive. And vice Versa, if we were to put them in a location where's there's loads of rocks mostly grey and black rocks, the brown one would die out since prey can spot it easier. So that's what that means. I'm a bio major, hope I helped. :)