In the culinary arts, the word marbling refers to white flecks and streaks of fat within the lean sections of meat. Marbling is so named because the streaks of fat resemble a marble pattern.
Also called intramuscular fat (or sometimes intermuscular fat), marbling adds flavor and is one of the main criteria for judging the quality of cuts of meat. In general, the more marbling it contains, the better a cut of meat is.
Also see: What is the Best Steak