In the United States, the standard placement of the eye chart is on a wall that's 20 feet away from your eyes. Since many eye doctors' offices don't have rooms that are 20 feet long, in a smaller room the eye chart may hang behind the patient chair, using mirrors to make it appear in front of you at a simulated distance of 20 feet.
20/20 vision (or really, 20/20 visual acuity) is considered "normal" vision, meaning you can read at 20 feet a letter that most human beings should be able to read at 20 feet.
Eye charts can be configured in various ways, but generally, if during an eye test you can read the big E at the top but none of the letters lower than that, your vision is considered 20/200. That means you can read at 20 feet a letter that people with "normal" vision can read at 200 feet. So at 20/200, your visual acuity is very poor.
In the United States you are considered "legally blind" if your best-corrected visual acuity (meaning, your best distance vision with eyeglasses or contact lenses) is 20/200 or worse.
To get a driver's license in most of the United States, your best-corrected visual acuity must be at least 20/40.
Usually the 20/20 line of letters is fourth from the bottom, with 20/15, 20/10 and 20/5 below that. Not many people have 20/10 or better visual acuity, but many animals do, especially birds of prey, which have been estimated to have an acuity of 20/5 or even better.