20/20: Normal vision

20/40: Near normal

20/80: Moderate impairment

20/200: Legal blindness threshold

20/400+: Severe impairment

20/20 means you see at 20 feet what a normal eye sees at 20 feet

20/40 means you must be at 20 feet to see what a normal eye sees at 40 feet

Lower numbers after the slash indicate better vision