Mono and diglycerides are commonly used to stabilize emulsions and to thicken or foam oils. Unlike many ingredients, they have to be dissolved in oil, not in water. Once you heat the oil to above 60°C (140°F), the mono and diglycerides melt and can easily be stirred in.
They are high stability emulsifiers composed of monoglyceride and diglyceride taken from the fats of glycerin and fatty acids. Despite commonly being called "glycerin flakes", they do not actually contain any glycerin.
You can find out more about mono and diglycerides from my how to use mono and diglycerides guide or any of the mono and diglycerides articles and recipes below.