Vision (also called visual acuity) is tested by reading an eye chart at a distance of 20 feet. By examining many people, eye doctors have decided what a "normal" human should be able to see when standing 20 feet from an eye chart.
If someone has 20/20 vision, that means he can stand 20 feet from the chart and can see what a "normal" human sees. Therefore, if someone has 20/20 vision his vision is "normal" - a majority of people can see what he can see at 20 feet.
If someone has 20/40 vision that means when she stands 20 feet from the chart she can only see what a normal human can see if standing 40 feet from the chart. In other words, if a person with "normal" vision stands 40 feet away from a chart, and a person with 20/40 vision stands only 20 feet from the chart, both can see the same detail.
Likewise, 20/200 means that when a person stands 20 feet from a chart he can only see what a normal person standing 200 feet away can see. The cutoff for legal blindness in the United States is 20/200.
It also is possible to have vision better than normal. A person tested to have 20/10 vision can see at 20 feet what a person with normal vision can see when standing 10 feet from the chart.