This term indicates that an individual has a visual acuity of 20/20.

It's a term only used in North America - in Europe, 6/6 notation is used. These figures are based on letter charts which are used in the standard sight test, such as the Snellen chart.

The top number refers to the distance at which the chart is viewed (20 feet or 6 metres) and the bottom number refers to the distance at which a person with ideal vision can see a letter clearly.

So if you have 20/40 (or 6/12) vision then you'll just be able to see something from a distance of 20 feet that a person with perfect eyesight will be able to see from 40 feet.