You can say "fair enough" as an informal response in a conversation/debate or as an answer to a question when you want to acknowledge that the other person's argument/answer makes sense, and you are willing to accept it and surrender your position, or simply that you accept their answer. It means something like, 'Ok, you're right' or 'Ok, that makes sense' -- hence, "fair enough" = "your answer is fair enough for me to accept."
Example 1:
Person 1: Do you want to go out for burgers tonight?
Person 2: No, we have spent too much on restaurants this week already, and we have leftovers in the fridge that we need to eat.
Person 1: Fair enough.
Example 2:
Person 1: Would you like some cookies?
Person 2: No thanks, I'm trying to watch my sugar.
Person 1: Ok, fair enough.