People who are able to look back on the past and understand what happened have hindsight. If you go skating on a frozen lake and it cracks, in hindsight you'd know you should've paid attention to the giant "danger" sign.
Hindsight is like looking behind you to see what just happened (behind sight, get it?). Another way of describing retrospection, hindsight is a useful skill that can be cultivated. Hindsight often refers to a lesson learned from something going wrong. Billy Wilder, the American movie director, once commented wistfully, “Hindsight is always twenty-twenty.” It's much easier to see clearly after something happened than before.