What does it mean if your vision is 20 40?

A person with 20/20 vision can read an eye chart at a distance of 20 feet. A person with 20/40 vision can't and has to get closer to the eye chart. A person with 20/20 vision can read at 40 feet what a person with 20/40 vision

Answer

If you have 20/40 vision, it means that when you stand 20 feet away from the chart you can see what a normal human can see when standing 40 feet from the chart. That is, if there is a normal person standing 40 feet away from the chart and you are standing only 20 feet away from it, you and the normal person can see the same detail. 20/100 means that when you stand 20 feet from the chart you can see what a normal person standing 100 feet away can see. 20/200 is the cutoff for legal blindness in the United States.