The Advisory Committee on the 2015 Dietary Guidelines had this to say about fiber:
A growing body of evidence suggests that fiber may play a role in preventing coronary heart disease, colorectal and other cancers, type 2 diabetes, and obesity.
Because the average intake levels of dietary fiber are half the recommended levels, achieving the recommendation requires selecting high-fiber cereals and whole grains and meeting current recommendation for fruits and vegetables.
Fiber is a non-digestible carbohydrate. Only plant foods provide fiber; animal foods contain little to no fiber. Whole grains, beans, legumes, fruits and vegetables are good sources.