20/20 does not necessarily mean perfect vision. 20/20 vision only indicates the sharpness or clarity of vision at a distance.
In Europe, people don't say they have 20/20 vision, they say 6/6 vision, due to the fact that they use the metric system and 20 feet is equal to 6 meters.
If you have 20/20 vision, you can see clearly at 20 feet what should normally be seen at that distance.
If you have 20/40 vision, it means that when you stand 20 feet away from the chart you can see what a normal human can see when standing 40 feet from the chart.
If you have 20/100 vision, it means that you must be as close as 20 feet to see what a person with normal vision can see at 100 feet.
There are other important vision skills, including peripheral awareness or side vision, eye coordination, depth perception, focusing ability and color vision that contribute to your overall visual ability.
Snellen eye chart:
The classic example of an eye chart is the Snellen Eye Chart, developed by Dutch eye doctor Hermann Snellen in the 1860s. There are many variations of the Snellen eye chart, but in general they show 11 rows of capital letters. The top row contains one letter (usually the "big E," but other letters can be used). The other rows contain letters that are progressively smaller.
In the United States you are considered legally blind if your best-corrected visual acuity (meaning, your best distance vision with eyeglasses or contact lenses) is 20/200 or worse.