what does each number of you vision rating stand for?20/20?

Answer 1

The first number says how far away to stand (usually is 20 feet). The second is the distance at which you should be able to see at that letter.

So 20/20 means that at 20 feet you can see the size letter that is normally visible at 20 feet. If your eyes are very bad you might have 20/400 meaning that when you stand 20 feet away you can only see the size of a letter that is normally seen at 400 feet. If you are far sighted you might have 20/10 meaning that you can see at 20 feet what people normally have to stand at 10 feet to see.
To fit the test into a smaller office they might actually conduct it less than20 feet away and make the numbers scaled smaller to account for the shorter distance. But the results still work the same way - can you see at a certain distance what normal vision people can see at that distance?.