"Hindsight is 20/20" means that when looking back at a decision, it is easy to see what the correct course of action should have been. "Hindsight" refers to the act of looking back, and "20/20" refers to perfect vision.
New Yorker magazine attributes the first use of the phrase "hindsight is always 20/20" to Billy Wilder, a prolific filmmaker in the 20th century. Variations of the phrase have become a proverb. The statement is frequently made after a person is criticized for a mistake. The person responds, "Hindsight is 20/20," as a way of pointing out that it is easy to see the correct path after the fact.