Think of it like this:
* 20/20 vision is perfect eyesight. You can see clearly and distinctly.
* Hindsight is looking back at the past.
So, the saying implies that when we look back, we can see things much more clearly than we could when they were happening. We can understand the consequences of our actions, the mistakes we made, and the better choices we could have made.
It's often used to express the idea that:
* It's easy to criticize or judge a situation after the fact.
* We're not always able to make the best decisions in the moment because we lack all the information.
* We learn from our experiences, even if those experiences were difficult.
Example: "I knew I should have studied harder for the test, but hindsight is 20/20."