20/20 is the same as 6/6. it's something to do with seeing a "size 6" letter at 6metres, or 20feet. I'm not sure on the "size 6" part, but i know the distance is 6metres or 20feet. There are guidelines on the CAA website, but i think 20/20 is the minimum, i wasn't sure myself, so i printed the webpage off, took it into the otpicians and she checked my sight against the guidelines and said i wasn't even close! I was so pleased.
You can get better than 20/20 vision, which is normal vision. You can have perfect vision which is i think 6/4, where you can see a "size 4" from 6metres, or you could see a "size 6" from a larger distance etc