hindsight is 20/20

If someone says, "hindsight is 20/20" they mean that it's really easy to see what you should have done when you look back. It's just impossible
to have known at the time what to do when you made the decision.
If you bought a stock where you lost 30% of your money, you could say, "I really wish I didn't buy the stock, but at the time it seemed like a good idea. Hindsight is 20/20". Just so you know, perfect vision or "eye sight" is 20/20 vision. Hindsight is looking back in time. So, you can see clearly what you should or shouldn't have done once you are already in the future. Does that make sense?