Explain darwin's theory of natural selection?

 "More individuals are produced each generation that can survive. Phenotypic variation exists among individuals and the variation is heritable. Those individuals with heritable traits better suited to the environment will survive." -https://www.google.com/search?q=Darwin%27s+theory+of+natural+selection&oq=Darwin%27s+theory+of+natur...
Basically what that means hun is that Darwin believed in a sort of adaption. That each generation was better adaptable to the environment and those who were healthier "survival of the fittest" would be more likely to survive. ^.^ Hope this helps!