Here's how it works. Imagine you're a caveman or cavewoman alive 100,000 years ago — and you come face to face with a hungry saber-toothed tiger. You have two choices: 1) Run for it (that's flight), or 2) pick up your club and battle with the tiger (that's fight). A final choice (be eaten) doesn't seem like such a good one!
Today, you can apply fight or flight to that bully who confronts you and won't listen to reason. You have two choices: 1) Turn and walk away (flight), or 2) fight, even though you know fighting won't solve the problem.