Fiber, very simply, is the part of your food that you and your good bacteria and other microbes cannot break down and use (digest). Since this part of your food cannot be digested and used, it will stay in your intestinal tract to be passed out of your body when you have a bowel movement. As the fiber accumulates in your large intestine (colon), it help fill the colon which presses on the wall of the colon. This pushing on the wall of the colon, ultimately, causes the colon to squeeze moving everything towards the anus creating a sensation to go have a bowel movement.
Fiber Needs Help
The fiber needs the help of good intestinal microbes to hold onto enough water to keep your stool (also called feces and manure) from drying out. Without these good intestinal microbes that live in the gut flora, everything sits in the colon for too long causing too much water to be removed. This leads to constipation.