The aperature is essentially the hole in your lens that allows the light onto
your media. The aperature is also known as the f/stop. The smaller the f/stop
is, the larger the diameter of this hole is; and vice versa the larger the
f/stop is, the smaller the diameter of this hole is. This is due to the relationship
that determines the f/stop. It is the focal length of the lens
(how "wide" or "long" a lens is) divided by the diameter of this hole. You
adjust your aperature based on the amount of light available on your subject,
and how much depth of field you want.