If I recall correctly, Jake makes this statement about the Greek Count that he and Brett meet and pal around with for awhile. While the Count is infatuated with Brett, instead of being jealous and controlling like all of Brett's other men, the Count appreciates Brett for the free spirit that she is, and he enjoys being with her. He enjoys buying Jake and Brett expensive dinners and champagne and he does not feel like he is being taken advantage of because he wants to do it. He is wealthy and knows what quality is. He recognizes and enjoys good cigars, good wine and good food. In this, the Count is a foil to the other characters in the novel because his money is actually bringing him pleasure, unlike the other lost characters in this depressing novel.
So, what Jake means is that the Count knows what quality is, does not mind paying for it and enjoys it when he gets it. This is living. The count is one of the few characters in this novel that enjoys life.
So much of Jake's discussion and the way that he comes to view things in the two chapters as he thinks about his life and his friends and his relationships is about this "it" and learning to be happy with it, whatever it is.
Rather than some kind of object or a certain amount of money or some kind of posession, this it is about one's lot in life or the way that they come to terms with what they do or don't have. It is important to be satisfied with your "lot" in life rather than be constantly looking for something else. Jake comes to sort of codify the idea that such a big part of it is simply learning to be comfortable with "it."