Soy lecithin is a fatty substance extracted from soybeans. It is often a byproduct of soy oil production. Soy lecithin is used as an emulsifier and a stabilizer. It's effective at binding fat-based ingredients (like oil) with water-based ingredients, and so it is used to emulsify salad dressings and other ingredients where oil and water are mixed. It is also used to create light and airy foams.