Thanks for the feedback, everyone! 

20/20 is a measurement referring to a person's eyesight, their ability to see clearly. I always thought it meant "perfect" vision, but apparently it is only how clearly a person can see from 20 feet. Perhaps *this* is why it is mostly understood only by North American speakers, because it uses feet rather than metric measurement. 

The expression, "Hindsight is 20/20" is something we often say to describe clarity and understanding about an event or situation that occurred in the past. 

Dialogue example:

-- I want to play the guitar, but private lessons are so expensive! I realize now that I should have taken advantage of free music classes in high school. 

-- Yep. Hindsight is 20/20.

or

-- A lot of people don't understand how detrimental drinking and driving can be until they get caught.

-- Yep. Hindsight is 20/20.

https://www.aoa.org/patients-and-public/eye-and-vision-problems/glossary-of-eye-and-vision-conditions/visual-acuity

Mi Sha,

Hindsight is 20/20... it's idiomatic for English speakers.  In hindsight, things are obvious that were not obvious at the beginning.  One is able to evaluate past choices more clearly than at the time that they made the choice.  20/20 refers to perfect vision.  So... it's like saying that we see perfectly once we know the outcomes but we wish that we could have seen those outcomes perfectly when we made the choice.

Some people have better than 20/20 vision.

My vision is nothing remotely close to 20/20.  Back when I was in college, I remember going to the eye doctor.  The doctor's assistant must have missed the class in college about dealing with a patient in an appropriate manner.  She looked at the reading of my vision, and she said to me,

"My God!   I'd hate to run into you on the highway without your glasses on!"