"Fair" can mean "equitable." A 200-pound boxer fighting a 140-pound boxer is not a fair fight. Two 160-pound middleweight boxers fighting is a fair fight.
"Fair" can also mean "acceptable." One school grading system uses the words "excellent," "good," "fair," "poor," and "failing." Notice that in this context "fair" is not as good as "good."
"Fair enough" means that the speaker has decided that some proposal, argument, or explanation is acceptable.
A: "How can you vote for X? X lies all the time."
B: "Your candidate, Y, has lied, too, for example about thus-and-such."
A: "Fair enough. I see your point. It's valid. I still think X lies much more than Y, but I agree that they both lie."