Fiber can be found in many things from wheat breads, fruits, and vegetable to produce, almonds and legumes. Here is what I understand about them. They come in two types, soluble and insoluble.
Soluble fiber is found in the core of an apple or the softer inner part of grains. These turn into gas in the intestinal track which aids in the digestion of other foods and slows the rate your body absorbs glucose. So this stuff helps get more out of your food while balancing out energy boosts.
Insoluble fiber is found in stuff with more strength to it like whole wheat, rice or broccoli type foods. This does not dissolve in water and is beneficial with digestion and absorption.