The Insulin is secreted by pancreas which is a long, flat gland in your belly that helps your body digest food. Insulin is kind of like a key that opens the doors to the cells of the body. It lets the glucose in. Then the glucose can move out of the blood and into the cells. But if someone has diabetes, the body either can't make insulin or the insulin doesn't work in the body like it should. The glucose can't get into the cells normally, so the blood sugar level gets too high. Lots of sugar in the blood makes people sick if they don't get treatment. So they have to be given external insulin.