The answer is a process that selects variation that help survival and that spreads the variation to more offsprings.
The idea is that there is a higher chance of survival if the organisms has certain characteristics and it is more likely to pass on their genes of this characteristic to the next generations.
For example, there's dark colored fish and white fish. In the deep ocean, dark colored fish can camouflage themselves. However, white fish can be seen easily by predators. Therefore, eventually more dark colored fish survives and more offsprings are dark ones.
