When we talk about vision testing, we usually consider people reading eye charts. A visual acuity of “20/20” means that we expect someone to be able to see that part of the chart from 20 feet, and we think that that person can see the chart from 20 feet. So, if the second number (the “bottom” number of the fraction) is higher, that means the vision is worse. For example, “20/400” means that we expect the patient to be able to see the chart from 400 feet, but that person can only see the chart from 20 feet away. Conversely, if the second number is lower, that means the vision is better.
There are many, many people who can achieve visual acuities of better than 20/20. It’s not uncommon at all for me to see patients with 20/15 vision. What’s the best possible vision? I recall learning in residency that the theoretical best visual acuity is ~20/8 for humans. That would be more than twice as good as what we normally consider good vision at 20/20!