A verbal way of confirming that you have understood that happiness isn't always about how easy your job is or how much it pays. It's about how you feel about what you are doing at the moment and expressing that "this is where it's at."
It also communicates to another that you feel strongly about this and that if he can enjoy the moment it is also ok for the other person to enjoy it. Sometimes it is an introduction for the person who says "Fuckin' A" to the new type of enjoyment that may have gone unnoticed in the mind of the listener.
An office worker loses his job after the place he works at burns down. He then works for the contruction company who does the cleanup. Typically, it is thought the office worker has the better job because it is easier and pays more. In this short excerpt from the movie "Office Space," a lot is said in a little amount of time with the two words.
Peter: This isn't so bad, huh? Makin' bucks... gettin' excercise... working outside?
Lawrence: Fuckin' A.
Peter: Fuckin' A.