18/18 and 20/20 are both the same, 20/20 means that you can see something 20 feet away that an average person with 20/20 vision can also see 20 feet away, if someone has 20/15 vision it means they can see something 20 feet away that a person with 20/20 vision can only see 15 feet away
It means it’s better than 20/20. 20/20 is great, but numbers on the right that are smaller than the left are even better. I had 15/15 vision. The results are the same. It just represents how far the chart was set up. But say it was 20/15 vision. That would be excellent.
It means your doctor uses a different standard of measurement than the norm. He’s making it seem like your eyes are better. Essentially, You see at 18 feet, what a normal person can see clearly at 20 feet. And that’s all it means. Glasses are really optional for you. Not absolutely necessary but you may need them eventually.