If an eye doctor tells you that you have 20/20 vision, it means that you meet the standard of visual acuity at a distance. This is measured through a test in which you are asked to read letters of a certain size on a chart simulated as 20 feet away.
Visual acuity at a distance is only one measure of how well you can see. Other important visual functions include peripheral vision, near vision, contrast sensitivity, color vision and depth perception.