20/10 great vision
A person who has 20/10 vision
sees an object at 20 feet that someone with "average" vision will only see clearly at 10 feet (better than average vision).
With 20/50 vision, in many states you are not legal to drive unless it can be corrected. In other words a person with 20/50 vision will see at 20 feet what a person with average vision will see at 50 feet (worse than average vision).