The term 20/20 (or 6/6) vision comes from the fact that the eye test chart is intended to be placed at a standard distance away from the patient being measured for visual acuity – 20 feet in the United States, and 6 metres in Europe. The numerator refers to the distance at which the person being tested can perceive the detail, whereas the denominator refers to the distance at which a person with normal visual acuity can perceive the detail. Thus, 20/20 vision means that you can see the same detail 20 feet away as a normal person. 20/40 vision, by contrast, indicates that you can see detail at 20 feet that a normal person can perceive from 40 feet away.
If you have 20/20 vision, then you should be able to discriminate an optotype (most often a letter) on an eye chart with features and patterns separated by a visual angle of 1 minute of arc. A minute of arc is an angular measurement equivalent to 1/60th of a degree.
The largest letter on the chart – most often the letter E on the Snellen Eye Chart, represents 20/200 (or 6/60) vision. If you cannot distinguish this letter using the best possible corrective lenses, either glasses or contact lenses, then you are by definition legally blind.
If the testing room is not large enough to accommodate 20 feet or 6 metres of distance between the chart and patient, a reverse chart can be projected in the mirror to double a 3 metre distance, or the size of the chart can be halved. It is also possible for humans to achieve better than normal vision without the aid of technology such as binoculars, though 20/10 is generally regarded as the maximum achievable unaided visual acuity. It is believed that some birds, such as hawks, have visual acuity of approximately 20/2 relative to humans.
While a simple internet search yields an array of do-it-yourself eye tests, the best and most accurate way to assess your vision – both in terms of visual acuity, and eye health – is to see an optometrist for regular eye tests.