Fiber consists mainly of cellulose and other indigestible complex carbohydrates.
It has no caloric value, and provides no nutrients.
However it’s still very important for 2 reasons:
It regulates digestion, especially in large intestine, allows normal bowel movement, improves consistency of stool, prevents diarrhea and constipation.
It slows down absorption of sugar and other nutrients as well. It’s a very important function, because we have more use of nutrients when they are absorbed slowly. Also it prevents sudden spikes in blood sugar level.
The only sources of fiber are plant foods including: fruit, vegetables, whole grains, legumes and nuts.
Making fruit juices eliminates the fiber from fruit and speeds up sugar absorption which is not good. Occassional juices are OK, but we should be moderate with it. Instead of juices, smoothies are better, because they don’t eliminate the pulp and the fibers within fruit.

Eating whole raw fruits and vegetables is always the best option.