The undigested substances are passed out of the body through the

The undigested substances are passed out of the body through the digestive system. When food gets into the body through the form of chewing, it travels through the mouth, esophagus, and into the stomach; as the stomach builds its gastric acids responsible in breaking down food, the food particles travel through the small and large intestines where nutrients will be absorbed by the body. 

Anything that is undigested will be secreted out of the rectum. Usually plants are the most indigestible substances that the body intakes. These materials that can't be digested help/aid in pushing other materials that are inside the intestines. This is what is colloquially known as "dietary fibers" that keep the digestive system healthy and clean of excess undigested food/debris.