THE APERTURE is built inside each lens and it controls the light entering a camera. A lot like the shades on a window control the light:
Aperture is measured in f stops. The larger the number under the f, the smaller the hole (aperture) and the less light that comes in.
A wide open aperture results in a shallow depth of field whereas a smaller aperture results in a great depth of field. If you want to know why this is see our lesson on the Physics of Depth of Field.
If you’re interested in knowing what the “f” is about in all the aperture settings see our lesson on “Aperture F Number“.