A selecting agent is any factor, environmental or otherwise, that affects fertility or mortality. Selecting agents include available food sources, local predators, and many other factors in or around a biosphere or individual.
An individual organism's fertility is a very basic example of a selecting factor. If an organism cannot reproduce, or if its reproductive faculties are limited, it stands a greater change of having its unique genes peter out due to failure to reproduce. If an organism is highly fertile, then it stands a good chance of passing on its genes and thus its fertility.
Both a predator's ability to hunt and a prey animal's ability to avoid capture are selecting factors for the same reason. Other, more complex selecting factors include an organism's ability to adapt to changing circumstances. These circumstances can include sudden shifts in climate, differing availability of food and the introduction of new species through forced migration or other means.
Natural selection ensures that in the absence of superabundance, only the most suited organisms will survive to reproduce. Moths that cannot disguise themselves to avoid predators will naturally be eliminated in an environment where they are forced to compete with camouflaged moths for resources and space.