Carbs are like Lego blocks. The singular blocks are called "sugars" and include things such as fructose (fruit sugar) and blood sugar.
When fructose or glucose enter the digestive system, they are readily processed and enter the bloodstream as glucose. Here, cells take up as much glucose as they can, which can turn into a problem if decades-long oversaturation continues unabated. The only way cells can block glucose from entering is by becoming resistant to a key that unlocks the entryway, insulin. Thus, diabetes develops.
In any case, carbs can become extremely long chains, forming things such as chitin, the sturdy outer carapace of insects. If you ever failed to squash a cockroach, now you know why.
A type of these complex sugars aka polysaccharids is called "fiber". Again, this includes things such as cellulose, which you can see if you tear apart a piece of paper, or lignine, which are the strands seen when you break a branch off of a tree.
We can't fully digest this fiber, but there are herbivores and especially ruminants who can. For example, cows have a special digestive system consisting of several stomachs, where they chew some plants, digest them, chew them again, digest some more and so on.
Now for why that's important for humans. In our digestive system, the dietary fiber becomes partially digested, turning into a soft gel that travels down our 30-foot digestive tract and helps push things along. To date, the entirety of our medicine hasn't created a medication that cleans the digestive system with same efficiency as dietary fiber. Cats and dogs have a relatively simple digestive tract; they can live off of turned salami but we can't.
Also, dietary fiber swells up, giving you a feeling of fullness and slowing down the rate of absorption of sugars. You see, now the digestion actually has to work to extract food, which reduces the onslaught of sugar into the bloodstream.
The problem is that dietary fiber makes the food harder to cook, chew and digest. And boy, do food companies hate that. Dietary fiber reduces the throughput of customers. Why would a fast food company want you to eat a hamburger for 30 minutes? They want you chewing that badboy in 2 minutes, out the door and then back right away, since food without dietary fiber does not satiate. Do you see the situation?
Source: dietary writer