20/20 isn’t perfect vision. It’s “normal” vision. The standard eyesight measurement of 20 divided by some other number is supposed to indicate how far away something looks to you if it’s only 20 feet away. Someone with 20/20 vision could look at something 20 feet away and see it as well as a “normal” person would expect to see something 20 feet away. This is kind of a clunky way to describe it, but it makes a little more sense when you compare someone with worse than 20/20 vision. If you have 20/40 vision, you can look at something 20 feet away and it would look as blurry as something 40 feet away would look to a “normal” 20/20 person.
Naturally, the coverse is also true. People can have 20/15 or 20/10 vision. When they look at something 20 feet away, they see it as clearly as something a normal person would see at only 15 or 10 feet away. The number 20 is, of course, completely arbitrary. Using the metric system, the “normal” eyesight would be 6/6. That is, something that is 6 meters away looks like it’s 6 meters away.
Read more about 20/20 vision at How Stuff Works.