APERTURE

Think of your lens aperture like that of your eye. When there is too much light, you squint and close the pupil's aperture.

f/22 is the most closed that most lenses will allow.

When there is low light, you dilate your pupil's and you can see better in the dark.

f/2.8 is the most open that most professional zoom lenses will allow.

Since the aperture of your lens directly affects the depth of field for your photographs, there is a separate page describing this function.