According to How Stuff Works, 20/20 vision means that a person can see what a normal person can see when standing 20 feet away. A person with 20/13 vision is above average because it means he can see at 20 feet what a normal person can see at 13 feet.
According to How Stuff Works, 20/20 is considered the norm for human eyesight. However, the higher the second number in this fraction goes, the weaker a person's eyesight. For example, if a person has a measured eyesight of 20/200, he or she is determined to be legally blind. However, the lower the second number, the stronger that person's vision is. A person with 20/10 vision would have exceptionally acute eyesight.