6/6 is the same as the old imperial 20/20.
It means that the person can see detail at 6 metres distance what a person with normal eyesight can see at 6 metres distance (the imperial 20/20 means 20 feet).
6/9 (the limit) means that the person can see detail at 6 metres distance what a person with normal eyesight can see at 9 metres distance - so the eyesight is worse.
6/5 is better than normal eyesight (you can see at 6 metres what a person with normal eyesight can only see at 5 metres) - so the eyesight is better than normal.
More info on Wikipedia as usual!
Visual acuity - Wikipedia, the free encyclopedia