6/9 means that vision is impaired - a person with 6/6 vision could read a certain line on an optician's chart at 9ft feet away, but a person with 6/9 could only read it at 6 feet away.
6/12 means that on the same chart, a person with 6/12 vision could only read a line at 6 feet away that a person with perfect vision could read at 12 feet away.