What it is
Sorbitol is a sugar that occurs naturally in berries, cherries, apples, pears, plums, algae and seaweed. It can also be produced synthetically, but the type used in cosmetics is generally derived from corn.
What it does
Sorbitol is a humectant: it draws water from the air into the skin, thus helping to hydrate it and preventing moisture loss.
In addition, it is also used as a thickening agent.
In an arid climate, Sorbitol can draw water out of the skin, dehydrating it more. However, most products are formulated to avoid this from happening.