Also called glycerol or glycerine, glycerin is present in all natural lipids (fats), whether animal or vegetable. It can be derived from natural substances by hydrolysis of fats and by fermentation of sugars; it also can be synthetically manufactured.
Glycerin is a skin-replenishing and restoring ingredient, meaning it is a substance found naturally in skin. In that respect it is one of the many substances in skin that helps maintain a healthy look and feel, defending against dryness.
Humectants such as glycerin have always raised the question as to whether or not they take too much water from skin. Pure glycerin (100% concentration) on skin is not helpful and can actually be drying. So, a major drawback of any humectant (including glycerin) when used in pure form is that it can increase water loss by attracting water from the lower layers of skin into the surface layers of skin, where the water can easily be lost to the environment—that doesn’t help dry skin or any skin type. For this reason, glycerin and humectants in general are always combined with other ingredients to soften skin. Glycerin combined with other emollients and/or oils is a fundamental cornerstone of most moisturizers.
Research shows that a combination of ingredients, including glycerin, dimethicone, petrolatum, antixoxidants, fatty acids, lecithin, among many others, are excellent for helping replenish skin.
References for this information:
International Journal of Cosmetic Science, August 2016, ePublication
British Journal of Dermatology, July 2008, pages 23-34
Journal of Cosmetic Dermatology, June 2007, pages 75-82
Proceedings of the National Academy of Sciences, June 2003, pages 7,360-7,365