The fight or flight response is an unconditioned response (UR) (or the unlearned, natural response) that occurs in response to a danger or threat in the environment, which is the unconditioned stimulus (US).
Now suppose you paired the unconditioned stimulus (US) (danger or threat) with a conditioned stimulus (CS), such as a bell ringing. Eventually, with repeated pairings of the danger (US) with the bell ringing (CS), then the presentation of the bell ringing alone (CS) will result in the fight or flight response, the conditioned response (CR).
So put simply:
US --> UR
US + CS --> UR
After multiple pairings of US + CS:
CS --> CR