Glycerin is a humectant, which means that it attracts and retains moisture on your skin. Think of it like a natural skin lotion in soap.
Glycerin is a natural by-product of the soapmaking process. Most commercial soap manufacturers remove the glycerin from soap, process it and then sell it to other industries where it's used in pharmaceuticals, foods, cosmetics and other products.
This isn't the case for handcrafted soap! Glycerin still remains in the bar. The end result is a mild, pure soap that gently cleanses your skin without being harsh, irritating or drying. (Information from HSCG)