Glycerin, also known as glycerol, is an extremely common substance in cosmetic formulas; especially in skin care. It is mainly used for its humectant properties; i.e., it takes the humidity that is in the air and maintains it in the skin. That makes it a perfect ally for moisturizing treatments (in skin care as well as in hair care).
In addition to its humectant properties, glycerin is sometimes used for its viscosity. In a balm, for example, it will aid in lessening the overall hardness in order to obtain a product that melts better. In a cream, it will give the final product a bit more body.
Last but not least, in a natural composition, like a plant extract for instance, glycerin will help boost preservation (that is, under the condition that it be used at a high level).
In order to reap the humectant benefits of glycerin, it must be used at a rate lower than 10%. Any higher and it will have the opposite effect. In other words, glycerin will tend to speed the dehydration process by taking humidity inside the skin or the hair and releasing it into the air.
In addition, the glycerin used in conventional cosmetics can be of animal origin. This version is cheaper to extract than vegetable glycerin (which is generally extracted from rapeseed). Unfortunately, it is impossible to know if the glycerin used in a product you have bought is from animal or vegetable origin; except in organic cosmetics where only vegetable glycerin is allowed.