It's common knowledge that 20/20 vision is as good as it gets. But where does this phrase originate?

Everyone knows what 20/20 vision means. It’s perfect! As opposed to what, though? How would bad vision be described on this scale? Does a higher or a lower number mean worse vision? Does each number refer to an eye? How does this scale work? Thank you, jelly who can give me the answer to the random question that’s been bugging me for days.

I started wondering about this in the shower the other day when I wasn’t wearing my glasses and I couldn’t clearly see the trees outside the window. Yes, I live so far in the country that I shower with the curtains open.