This means how vital and strong your muscles are. A lot of muscle mass does not necessarily mean that your muscles are still strong and vital. Conversely, less muscle mass in the body does not mean that the muscles are not strong and vital. 

Even within your own body, the quality of your muscles can differ. If, for example, your left arm has better quality muscles than the right one, an imbalance in your body is lurking. This in turn can lead to injuries. From the age of 18 it is possible to assess the quality of your muscles. This is assessed on the basis of the ratio of muscle mass to height. 

Read more about muscle quality here: https://tanita.eu/understanding-your-measurements/muscle-quality