Natural selection is a process which allows the evolution of species to better suit the life in their environment, and is the basis of Darwinism. It's based on the fact that organsims that are better suited for life in the environment will survive more and have more chances to reproduce, thus increasing the chance of their positive genes carrying on in their offsprings.
In order for natural selection to be possible, the species must fulfil these criteria:
- there must be variation in genes between the organisms
- these variations must be inheritable by the offsprings
- adaptation by Differential Survival
In essence: if there's some heritable variation in the population, and if that variation affects how well the organism can survive and reproduce, its possible for traits that are well suited for these causes to persist more into future generations and become more and more common.