20/20 Vision

... means that the patient's visual acuity is "normal".

On the Snellen chart, used in eye tests (the one with one huge letter at the top and lost of tiny ones at the bottom), one of the rows towards the bottom is designated as the smallest one that someone with "normal" vision should be able to read. If you can read one of the rows below this one, your vision is better than normal. If you can only read bigger rows, it's worse than normal.

"20/20 vision" means that your visual acuity is normal. The significance of the "20"s is that at 20 feet, you can read something that someone with normal vision can read at the same distance.

If your vision is 20/15, this means that at 20 feet, you can read something that a "normal" person can only read at 15 feet. If it's 20/30: what a normal person can read at 30 feet, you can only read at 20 feet. 20/15 is good; 20/30 is ... less good.

If you think of these figures as fractions: the higher the number, the better is your eyesight. Anything greater than 1 is good; anything less than 1 is ... less good.

For more details, I recommend allaboutvision.com.