"Fair enough" is a phrase that means "I don't completely agree, but I understand and accept your point." You use "fair enough" in situations such as:

• You invited a friend to your wedding. She says that she can't make it because she's going to be taking her final exams for law school that week. You want her to come, but agree that she needs to take her exams.
• Your brother wants to buy a new car. You don't think he should because you don't think he needs it. You argue with him about it, but he says that it's his decision. You agree that it's his decision, so you're going to stop arguing about it.

"Fair enough" doesn't sound too negative or too positive. It's a neutral, balanced expression. You can use it at work, with friends, and in most situations.