It measures Body Fat by electrical impedance. the thim metal strips on the surface are the contacts.
These scales can give you an idea of bodyfat but by no means are they the most accurate way or the most consistant way.
Many variables come into play such as the level of hydration of your body and wether or not the soles of your feet are wet IE just coming out of the shower.
If you test on these scales the bottoms of your feet should be damp and you should be at close to optimum hydration for the best readings.
Also there are some personal data you will have to input into the scale height measurements, sex, etc.
I own one and it gives a rough estimate pretty close but I have seen the fluctuations. Nice to have but hardly the best way.