What exactly does "20/20 vision" mean?
20/20 vision references a unit of measure of visual acuity, or ability to see detail. The best the average person is able to see detail is designated 20/20.
As the bottom number increases, visual acuity decreases. 20/40 vision means that a patient sees at 20 feet what a person not requiring a spectacle prescription sees at 40 feet. Another way of saying this is that a person with 20/40 vision needs objects at half the normal distance for him to see them clearly. 20/40 or better visual acuity is the standard cut-off to pass a Driver's License vision screening, without correction, in all 50 states. 20/40 best-corrected visual acuity is acceptable to drive without restriction on your driver's license.
As the bottom number increases, vision increases. 20/15 vision is better than 20/20. A person with 20/15 vision can see objects at 20 feet that a person with 20/20 vision can only see at 15 feet.
Legal blindness is defined by visual acuity of 20/200 or less, in the better seeing eye, with correction. Also included in the definition of legal blindness is a visual field of less than 20 degrees.