Glycerin is a humectant. It is an ingredient derived from plants that helps retain moisture in the skin. It also helps to draw water from the deeper layers of the skin to the outer layer, thus preventing the loss of moisture in the skin.
Glycerin is extremely hydrating and gives a long lasting effect of "plump skin" by boosting aquaporins production. (Aquaporins are proteins that help maintain the flow of essential hydrating subtances to the skin's outer layers).
It can also help to protect the skin from harsh products that can be found in the cosmetic industry. By attracting moisture, it can help the outer layer of the skin to stay hydrated and prevent the skin from being irritated during the cleansing process.
Glycerin is just an all-rounder, it can help all types of skin to reach their potential and it is great to help the skin stay hydrated and soft without being aggressive or harmful.