In a
camera, the f-stop number is the ratio of the length of the
lens to the
diameter of the
aperture. Higher f-stop numbers give you a greater depth of field. At the extreme end, a
pinhole camera will give you almost total depth of field.
On your fancier cameras with zoom lenses, the hyperbolae on the barrel of the lens correspond to different f-stop settings, and are to be used in conjunction with the focus to tell you the depth-of-field of a specific f-stop at that zoom level.