Perfect vision essentially, when someone can see without glasses, but also clearly without a sign of blur. Many people in reality do not need glasses and most have fairly good vision, but not everyone is applicable to the standard of 'perfect' or 20/20 vision. It means you can see clearly 20 feet away while someone without 20/20 vision would not be able to see that far that clearly
It means you can see 20 ft away, at the distance a normal person with perfect vision will see standing 20 feet away.
While many people think that 20/20 vision is "perfect" eyesight, it is actually just "normal" eyesight. Specifically, someone with 20/20 vision can see an object 20 feet (6 meters) away with the clarity that a "normal" eye will see an object at that distance. It's possible to have better than 20/20 vision. A person who has 20/15 vision, for instance, is capable of seeing an object that's 20 feet away with the same clarity that a normal eye will see an object that's just 15 feet (4.5 meters) away.
Source: Prevention's Giant Book of Health Facts, p. 519.