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.