Fibre is classified as a carbohydrate but does almost none of the things carbohydrates are known for. The difference comes from one structural fact.
The body lacks the enzymes to break it down
Digestible carbohydrates are chains of sugar units joined by bonds human enzymes can cut, releasing sugars that are absorbed in the small intestine.
Fibre is built from similar units joined by bonds those enzymes do not recognise, so the chain passes through the small intestine largely intact.
That is why fibre contributes little usable energy while still occupying volume and weight in a meal.
It changes the speed of everything around it
Some fibres dissolve in water and form a viscous gel, which slows how quickly stomach contents move onward and how quickly nutrients reach the intestinal wall.
Slower delivery means the sugars from the rest of the meal are absorbed over a longer period rather than arriving all at once.
This is why the same amount of starch eaten as a whole grain and as a refined flour does not behave identically once swallowed.
Insoluble fibre works mechanically instead
Other fibres do not dissolve and instead retain water, adding bulk and softness to what passes through the large intestine.
That bulk stimulates the intestinal wall and keeps material moving, which is a physical effect rather than a chemical one.
Most plant foods contain both types in different proportions, so the categories describe behaviour rather than separate ingredients.
Gut bacteria are the actual consumers
Fibre that reaches the large intestine meets bacteria that do possess the enzymes to break those bonds, and they ferment it.
That fermentation produces short-chain fatty acids, which the cells lining the colon use as fuel, and it also produces the gas that a sudden increase in fibre makes noticeable.
Because different bacteria prefer different fibres, the range of plant foods eaten matters alongside the total amount.
Processing is where fibre is usually lost
Milling separates the bran and germ from the starchy interior of a grain, and the fibre leaves with them.
Juicing removes the structural material of fruit while keeping the sugars, which is why the two forms of the same fruit are digested at different speeds.
Anyone managing a specific digestive or metabolic condition should work out the right approach with a clinician, since individual tolerance varies considerably.