"Fair enough" represents that the person isn't overwhelmingly in support, but considers it to be "okay".
Some examples:
A) How much do you want for your car?
B) $4500
A) Fair enough.
In this example, A considers the request (price) to be okay; not great.
A) I'm sorry I didn't mean to insinuate that you were a bad person. I just meant that we don't agree on some things.
B) Fair enough.
In the second example, you can read between the lines that B is not happy, but appears to be accepting the apology as "fair enough". B is probably still upset and just doesn't want to talk about the issue any longer.
As DavidK noted, it *can* mean that the person is okay with the point, but even in his example, the boss wasn't really happy as the boss followed up "fair enough" with the fact that s/he had been exited about getting the report. (Note the use of the word "just".)
As you can see, depending on the context, there's a lot that can be inferred from how "fair enough" is used, so use this expression carefully.