Can docs explain what does visual acuity indicate?

Vision standard: In the USA the standard is the Snellen fraction of 20/20. The upper number means the letters are 20 feet away. The lower is the distance the normal could be to read the number. If someone is 20/200 at best, the letter is large enough that the 20/20 normal could be 200 feet away and still read it. Visual charts have a graded series of letter size to get the best fit.