Insulin secretion is directly related to the amount of blood glucose that passes through the pancreas, which is responsible for releasing insulin. Insulin in turn binds it self to the muscle cell receptors and enhances (helps) the entry of glucose into the muscle cells.
Insulin is secreted when your blood sugar rises... when it is secreted it pushes the nutrients (glucose, amino acids...) into your muscles (or fat when you are sedentary)... so as you can imagine thats a veyr good thing.
Insulin is the most anabolic hormone in your body.