Its not part of the JAR Class 1 medical requirments. Its basically a test to see if you can read letters 6 feet away that someone with perfect vision could read at 24 feet.
This was taken from a previous post at PPRUNE:
6/24 vision means that you require 6 feet to see things that a person with normal vision can see at 24 feet. The following is extracted from a yahoo search on vision acuity:
"Visual acuity is expressed as a fraction. The top number refers to the distance you stand from the chart. This is usually 20 feet. The bottom number indicates the distance at which a person with normal eyesight could correctly read the line with the smallest letters. Normal vision is considered 20/20. If your vision is 20/40, the line you correctly read at 20 feet could be read by a person with normal vision at 40 feet."