There are many different answers to this question but I'm thinking it has something to do with the difference between visible fat that lies on the outside of the meat (you can see it raw and cooked, it's white in color) and can be easily separated (i.e. trimmed), and the fat that is infiltrated within the muscle fibres. You should choose lean meats which have very little fat. Certainly, some cuts of meat like ribeye contain a lot more fat than turkey for example.
