Is it true that fiber is digestable

The answer is false.

Fiber cannot be digested, as it is made up of plant matter or cellulose.
Cellulous acts like metal in our bodies; out digestive tract was not programmed to break down cellulous. As we evolved, we mostly fed off of meat. The substance acts as a 'cleaner' per say; whenever you eat excess amounts of salad, you will understand what I'm trying to say. The cellulous makes up the structure of your wastes, helping it to flow through your body more easily.

Hope this helps!

Fiber is not digestible. 

The only function that fibre serves is that it can add bulk to our food, thus it can stimulate the movement of our intestines. They tend to increase their size when they absorb water so that it can touch the wall of intestines to stimulate the movements. They're indigestible part of the food.  They cannot be digested therefore there isn't any nutrients or calories for us to absorb. 

However, fibre is necessary in our daily diet, because constipation may occur if we do not have enough fibre intake. 

Can you explain it well?