What does it mean when you have 20\20 vision?

Normal vision: It means that you can see at 20 feet away from an object, the same as an "average" person can see at 20 feet away from the same object.

Normal good vision: 20/20 means that you can see at 20 feet what an average/normal person can see at 20 feet. It's considered perfect vision although many people can see 20/15 and some even 20/10, which means that you can see at 20 feet what an average/normal person has to get 10 feet away to see.