Our scales use the Bioelectrical impedance analysis (BIA) method. It sends a tiny current allowing us to measure your impedance which can then be used to calculate an estimate of total body water (TBW). 
TBW can be used to estimate fat-free body mass and, by difference with body weight, your fat mass.

Note that:
Fat mass + Bone mass + Muscle mass = 100% of the body composition
As there is water in muscle and fat, the water mass cannot be added to the other metrics when calculating the body composition.