Everything in the body that is not fat: muscle, bone, organs and water, usually estimated rather than measured directly.
The arithmetic is simple: bodyweight minus fat mass. An 80 kg client at 20 percent body fat carries 16 kg of fat and 64 kg of lean mass. Track that instead of weight and a client who loses 3 kg of fat while gaining 1 kg of muscle sees progress on a scale that barely moved.
The catch is that every field method is an estimate. Calipers, bioimpedance scales and tape measurements all carry error, and bioimpedance in particular swings with hydration. What they are good at is trend, if the conditions are kept the same: same device, same time of day, same hydration state, same person taking the measurement.
Use the trend over four to six weeks and ignore any single reading.