You may not be from the US (or you might be and still have no idea, which is understandable since doctors are terrible at explaining it), so here's an explanation:
20/20 is effectively "normal" vision. Someone with 20/20 vision can see from 20 feet away what normal people can see from 20 feet. 20/50 vision means that someone can see from 20 feet away what normal people can see from 50.
To put it in perspective, someone with 20/200 vision is able to read the letters from a stop sign from 20 feet away, while normal people can see it from ten times that distance.
You want big words? I'll give you one. "Pneumonoultramicroscopicsilicovolcanoconiosis". I learned that in, like, sixth grade.