It's safe to assume that you've stumbled upon the expressions visual acuity and 20/20 vision. As frequently used as these terms are, do most people actually know their meaning?
The term 20/20 indicates the clarity and sharpness of eyesight from 20 feet away. When you have 20/20 eyesight, that basically means that from a distance of twenty feet you can see that which should be seen from that distance. You may not know this, but 20/20 vision is really just a standard measurement. Many people have eyesight better than 20/20; for example, some people have 20/15 vision, so what they could see at 20 feet, a person with normal vision might only be able to discriminate at 15 feet.
Each one of your eyes is examined separately. When you're asked to look at the eye chart and read out the letters, the smallest letters you can clearly read determine the visual acuity of the eye that's being tested.
However 20/20 vision doesn't always mean you have perfect vision, because, after all, it only indicates your distance vision. There are other really crucial vision skills; being able to focus on close objects, contrast sensitivity, peripheral awareness, depth perception, eye coordination and color vision - these all contribute to your overall ability to see. And actually, someone with 20/20 vision can still have unhealthy eyes. Those with damage to the nerves within their eyes from glaucoma, diabetes, high blood pressure, or numerous other diseases are still able to have 20/20 vision. This is why your eye care professional will always carry out a comprehensive eye exam, and not just a regular visual acuity test.
During your next eye exam, you'll know exactly why you're being told to read letters from the eye chart, and more!