From the Manufacturer:
Beta sitosterol is a plant-derived sterol, also known as a phytosterol. Plant sterols or phytosterols are common components of plant foods, especially plant oils, seeds and nuts, cereals and legumes.
Phytosterols are natural components of many vegetables and grains. Preliminary scientific evidence suggests that plant phytosterols may help to maintain cholesterol levels already within the normal range when consumed as part of a low cholesterol dietary program. The three main phytosterols in phytosterol complex are beta-sitosterol, campesterol, and stigmasterol. They are natural, safe and derived entirely from plant sources.