This is the estimated weight of bone mineral in your body. Healthy bones and a healthy bone mass are important for your body's strength, movement and load. This is necessary because you have an increase in bone mass up until the age of 30. After this age, the bone mass will slowly decrease. This does not immediately lead to problems, especially if you take care of a good balance in your body. Although it is unlikely that your bone mass will change within a short period of time, you would do well to check this with some regularity. Should the score be higher or lower than the given ranges from the beginning, this is not something you have to worry about. 

On this page you can read more about bone mass and average scores: