What does 20/20 vision mean?

This kind of notation usually refers to a persons visual acuity. The upper number refers to the general distance that the eye chart is placed (eg. 20 feet). The lower number refers to the distance at which a “normal” observer can see the same letters you can see at 20 feet.

Thus, if your vision is 20/40, this means that a “normal” observer sees the letter you see at 20 feet at 40 feet. Therefore, the higher the bottom number the worse the vision is from a “normal” observer.