20/20 means that a person has normal standard vision.
Basically, the second number stands for norm, and the first number shows a ratio to norm. 20/10 would mean: I can see at 20 feet what people on average can see from 10 feet.
20/15 is therefore better than 20/20, but it is not very common.
20/10 is even less common, but not all that rare.
20/8 is the best human vision ever recorded.
20/20 means normal standard vision