What is darwin's theory of natural selection?

Put simply Darwin's theory of natural selection states that the being that can best adapt to it's environment will continue to survive. That's what is meant by survival of the fittest.
An example a bug that can change colors to fit it's environment is better to hide from a predator that may eat it. Hence it will not be eaten when it's neighbor is easily visible. That bug will continue to survive to prolong the species.

that's a very simple answer to your question. Hope it helps