Food enters the upper part of the small intestine from the stomach. Here, digestive enzymes from the pancreas and liver are added to the mix in order to further break down the food and extract its nutrients. The absorption of these nutrients also takes place in the small intestine.
The jejunum is a lower part of the small intestine. By the time the food has reached this place it has been mostly digested by the enzymes that entered the upper part. If food were to bypass the duodenum (the upper part of the intestine) and come right to the jejunum, it would not be as broken down. Therefore, the person would not absorb as much of the fat or other nutrients in the food. This could be an effective weight-loss procedure.