Part of every eye exam is the viewing of the "Snellen chart" which most know to be the chart that has rows of text getting smaller with each line. Patients are asked to cover one eye and read this chart from twenty feet away. This chart exam determines if corrective lenses are needed; it determines how far away you are able to see objects with each eye. If a patient is able to read the smallest line, they are able to see clearly.
When brought into basic terms, 20/20 vision means: At 20 feet from the chart used at eyecare offices, a person with "normal" vision can read the smallest line of text with one eye closed. In essence, a person with 20/20 vision standing 20 feet away from an object is able to view it clearly. A person with 20/40 vision needs to be 20 feet away in order to clearly see what a person with 20/20 vision can see clearly from 40 feet away.
Does this mean that 20/20 vision is the best anyone can see? No, it does not. 20/20 vision is the normal standard. There is such thing as 20/15 vision where a person can clearly see something 20 feet away that a "normal" 20/20 vision person needs to be 15 feet away from in order to view clearly.