Fiber is a bulking agent. It also helps prevent our stomachs from absorbing sugars. Fiber is the skin on cucumbers, skin on apples, the cellulose in celery, the inside tissue of an orange, the bran on the outside of wheat or the covering on the outside of rice. It is indigestible material. It helps us get our bowels moving and has many beneficial properties.
Fiber has been implicated in improving digestion, reducing cancer risk, turning off oncogenes (genes that tell us to have cancer), making us feel fuller, faster, as well as many other beneficial properties. Of course, too much fiber will plug up our intestines, so it's a balancing act.
The Alo Diet encourages eating a lot of fiber. Natural sources of fiber are better than fiber supplements. Sure, we could put psyllium (Metamucil) on all our foods, but it'd be better to eat the lettuce, celery, and apples.
Fiber is not digestible. It passes through our bodies undigested. It also surrounds and binds carbohydrates, fat, and cholesterol and makes it difficult to absorb these substances. The Alo Diet recommends consuming fiber and taking advantage of it's beneficial properties.