It mean that you can look in the past (hindsight) and see what you did wrong. Perfect vision is 20/20. So as you look back, you're able to see "perfectly" what you should have done. "Hindsight is 20/20"
This the same question as "What does 'hindsight is 20/20' mean", which has already been answered.
20/20 vision is when you can see things without difficulty from 20 feet away. It's average vision.
It means less then 21 yet more than 19.20 refers to the power of the tractor not the age
It refers to the 2nd version/generation of the internet.
i.e., what's your location. It's Ham and CB radio talk