Your lean body mass is the amount of weight you carry on your body that isn't fat. The goal of any bodybuilder or fitness enthusiast is to drop weight while keeping your lean body mass the same, in other words, dropping your body fat percentage. To find your lean body mass, you must first know your body fat percentage.
Lean body mass does not necessarily indicate fat free mass. Lean body mass contains a small percentage of fat (roughly 3%) within the central nervous system (brain and spinal cord), marrow of bones, and internal organs. It is measured in terms of density.