This video pretty much explains everything about bioelectrical impedance.
Bioelectrical Impedance (Youtube)
Basically it measures the electrical impedance, or opposition to the flow of an electric current through body tissues which could be then used to calculate an estimate of total body water. Total body water can be used to measure fat-free body mass and, by difference with body weight, body fat. (Wikipedia)
I feel this is a good way of measuring body composition as it is non-invasive and is quite accurate. It can give you a variety of information that you or your doctor needs to know. However you need to adhere to certain requirements before you can test your body composition with the bioelectrical impedance.