They are “polymeric carbohydrate structures formed of repeating units joined together by glycosidic bonds” (from Wikipedia). In other words, these are naturally occurring polymers.
We can find quite basic ones like two monosaccharides like glucose and fructose joined together to make sucrose or D-galactose and D-glucose joined together to make lactose We can find more complicated ones like starch, glycogen, cellulose, xanthan gum, guar gum, hyaluronic acid, and so on. These tend to be used as hydrating ingredients, but most will thicken the water phase of our products.