Answer:
Natural selection means survival of the fittest, or the most fit animals for an environment. The most fit animals tend to thrive in their environment better than less fit animals. In this case, a short-neck Giraffe will not be able to reach leaves {its source of food}, causing it to be hungry and unable to eat. Over time, Giraffe's formed longer necks to be able to read tree leaves, making them more fit to survive.
If this is confusing, I can do my best to explain further! :D
