There are 2 kinds of fiber, soluble and insoluble. Insoluble fiber doesn’t dissolve in the stomach and can have rough hard bits that irritate the intestines as it passes through. Soluble fiber attracts water into the intestines and becomes a gel. Foods with a little soluble fiber can often be eaten in small amounts (depending on why you’re on a low-fiber diet) because the soft fiber gel doesn’t irritate the intestines the same way.