Insulin is a 'polypeptide hormone secreted by the islets of Langerhans and functioning in the regulation of the metabolism of carbohydrates and fats, especially the conversion of glucose to glycogen, which lowers the blood glucose level. ' Basically, it's made by part of the pancreas and is your body's way of dealing with glucose. As your blood sugar goes up, insulin is created, and works to allow your body to use up the glucose for energy, or store the left over as fat. It also takes part in shuttling nutrients to the muscles for repair. You can get a lot more detailed answers if you want, but for the purposes of my post earlier, it helps get your blood sugar down so you get into ketosis faster. Some people will take extragenous insulin to force the body to use up it's blood glucose faster, forcing them into ketosis faster than if you waited for the body to secrete the necessary insulin itself. Hope this helps.