Natural selection is the key process to the theory of evolution. It means certain traits of a plant or animal are either exaggerated or eliminated over time in a population based on the benefits of the trait versus the drawbacks of the trait. An example would be trees growing in a forest. Trees need sun to live. If this was a thick forest, the trees that grew taller faster would have more of a chance of living. This means their genetic material would more likely be passed on to the next generation of trees. Over time, trees in this thick forest would have a tendency to grow faster and taller.
Another example would be rabbits with brown or white fur. If there was a climate change creating a snowy environment, the white rabbits would benefit from blending in and would have less of a chance of being eaten by prey. Since more brown rabbits would be eaten before they could reproduce, the number of grown rabbits would slowly decrease overtime until there were only white rabbits.