Aperture,
1.7, 2.8, 4, 5.6, 8, 11, 16, etc.

The dictionary defines it as,
The opening in a photographic
lens that admits the light.
or
The diameter of the f-stop in an optical
system that determines the diameter
of the bundle of rays traversing in the
instrument.
Think of it as your eye. In dark places your eye
lens opens, for more light, in a brighter place
in closes.
The settings correspond to how much light is
coming through the lens. The lower number the more light.
The higher number less light.
With that information, and working with the
shutter speed ( which we will chat about another time)
you can adjust the lighting of the image you are taking.
An example would be like a day light picture. You would close
down the aperture to a higher setting which will allow less light.
Add a faster shutter speed and you could have a lighter image
that looks good.