The metabolism of an individual is shaped by its genetics, but just as significantly by its interactions with the environment. E.g. through food, lifestyle habits, diseases, medications and other factors.
This metabolism runs through many interconnected cascades - essentially providing energy and building materials for the body, from the conversion of food.
Due to these metabolic cascades, the many metabolic products of the human body are not independent of each other; they influence each other. The totality of these mutual influences (interactions) is called a biological network.
The comprehensive recording of the metabolic products in a sample thus represents, as it were, a digital image of the underlying metabolic cascades. Thus, the state of the organism at the moment of sampling is mapped in the best possible way - digitized, as it were.