What does 20/20 mean?  The top number 20 is always you; it’s always how far you have to stand from a letter to read it.   The bottom number is how far a “normal” person is able to read the same letter.   

So say you have 20/40 vision.  That means that you have to stand at 20 feet to see what most people can see from 40 feet. Or, in other words, you have to get twice as close as most other people to see the same image.  It also means it takes you twice as long to spot something when you’re driving.