The 20/20 Fallacy
Visual acuity (20/20, 20/40, etc.) is a useful eye test that's part of a complete eye exam. The first black and white letter chart to measure visual acuity was developed in 1862 by a physician named Snellen. That's why measurements like 20/20, 20/40, etc. are sometimes called "Snellen visual acuity" readings.
The top number in the Snellen fraction is always "20" and refers to the standard testing distance of 20 feet that's used to assess your vision. The bottom number is the farthest distance from which a person with "good vision" can see the smallest letters that you can see when you're 20 feet from them. Hence, if you have 20/20 vision, you have visual acuity equal to that standard observer, and you have good vision. If you have 20/40 vision, the smallest letters you can see at 20 feet can be seen by the standard "good vision" observer when they are 40 feet away. So your vision is only half as good as the standard.
Many people have eyesight that is even more acute than the Snellen standard of good vision (20/20). If you have 20/15 vision, you can see the smallest letters on the chart from 15 feet away, whereas the standard observer is at 20 feet. So your vision is better than the 20/20 standard.
The problem is that "20/20" visual acuity is not really "perfect" vision. The high contrast Snellen acuity chart (black letters on a white background) does not tell how sensitive your eyes are to different shades of gray. And some people with "20/20 vision" may be able to see the letters easily, while others with 20/20 vision struggle to make them out. The Snellen chart also cannot predict whether or not your night vision is disturbed with glare and halos, how easily your eyes tire, or how much light you need to read comfortably all of which are important components of your vision. Contrast sensitivity testing, however, can check for these visual acuity problems so that they can be corrected.
Be aware that even if your uncorrected visual acuity is 20/20 after refractive surgery, this does not guarantee your vision will be the same as the 20/20 vision you had with eyeglasses or contact lenses before surgery. It may be better; it may be worse.