"20/20" is visual acuity measured in Imperial units (feet), and "6/6" is the same standard measured in metric units (metres).
If you are 6/9, then you have to be 6 metres from a target to see it with the same clarity as someone with normal vision can see at 9 metres and so on....
Bottom line ... who cares what the numbers mean, just do what the previous posters have said; get to the optometrist, get your glasses or contact lenses and go strap an aeroplane to backside and start having fun!!
By the way, I'm the same as Best Speed - blind as a bat without my glasses but I'm still a pilot.