Muscle mass is a term for the bulk of muscular tissue in a person's body. There are three different kinds of muscle in the human body, but muscle mass almost always refers to skeletal muscle. Skeletal muscle is the most visible and directly contributes to strength and power.
Muscle mass includes smooth muscles, skeletal muscles and water contained in the muscles. Where the skeletal muscles are the most visible when there is no fat layer. Muscles consist of water and protein. This is why it's important to include protein in your adjusted eating schedule.