The Snellen fraction, named for the ophthalmologist who devised it, signifies one’s visual acuity at 20 feet (the top number) compared to standard vision
(the bottom number).
The standard, chosen by Snellen, refers to ordinary vision; it doesn’t mean perfect or even average vision. If you’re 20 feet from an eye chart and you see what eyes normally see at 20 feet, your vision is 20/20. (Each eye is measured separately and has its own fraction.) If you see what people typically must stand 15 feet away to see, your vision is 20/15. And if you must stand 20 feet away to see what ordinary eyes see at 30 feet, your vision is 20/30. The larger the fraction, the sharper the vision.
Source: The Cleveland Plain Dealer October 2012 by Marilyn vos Savant