Hindsight is a noun that refers to understanding a situation or event after it has happened. There's an English saying: "hindsight is 20/20." It is used mostly as a sarcastic term that means you can see something perfectly after you've already made a mistake trying to do it.

Hindsight is a word used to discuss an event in the past that the user speaks of that could've came out in multiple outcomes and about how an action had an impact on it. It could also be used to describe a formal of regret towards a previous action. "In hindsight, John taking that book really didn't matter"[more information would honestly be needed, but this is a quick reference.] "In hindsight, I should've asked her to come with me."