In 1859 Charles Darwin revolutionized our concept of the world and humanity's place in it with his book On the Origin of Species . Darwin explained the evolution, or change through time, of animals as resulting from a process he called Natural Selection . He modeled his hypothesis, in part, on the un-natural selection which we can all see around us.
People breed championship dogs, cats, race horses, etc.
If the individual passes, it survives because it has characteristics which make it better adapted to the local environment than other members of the same species. The important aspect of Natural Selection is not just survival, but the retention of beneficial characteristics within the population through reproduction. An animal which does not pass the test need not die, but it may just be too weak to successfully breed, so its characteristics are lost. We can define an adaptive pressure as any aspect of the environment which can affect the survival of an individual. Food availability, climatic variations, predators, etc., would all be examples of adaptive pressures.