Consider an example, a person X who's weight is say :90 kg.
Then consider another person who's weight is also :90 kg.
Both have same weight, BUT, X has more lean mass than Y. What does that mean?
it means your bodyweight is a sum of different factors:
- Your body fat mass (fat percentage)
- Your body muscle(lean) mass (muscle percentage)
- Your body water composition
Lean mass is thus the pure muscle mass without the fat. Men have more lean mass than women, because their body composition and physiology has more muscles, particularly in the upper body. Thus it makes the muscle's percentage higher than the one in women.
To calculate your lean mass you need first to measure your body fat percentage. You can do this using machines (some are not very accurate) or using a caliper. The caliper is the fastest and cheapest tool you can have.
Check out this video on how to use a caliper, and this link to enter your measured data and use the calculator to get your body fat percentage and lean mass.