The digestive systems main function is to break up food then prepare the energy, vitamins, and minerals it gets from food for absorption by the body.  However in the process of this the digestive system also makes a substance simply called waste.   Waste by definition is unneeded lost materials. The human body will remove all the nutrients and energy from food, but after all usable substance in the food is gone, the rest must be excreted.  If the task is not completed the digestive tract would quickly back up and the human body cannot survive.  See also excretory system.