When all systems are working properly, a hormone called insulin helps the glucose move from the bloodstream into the cells where it's burned for energy. The pancreas is the organ that makes and secretes insulin. Think of insulin as a "key" that has to unlock the cells to allow the glucose in. Insulin is supposed to bind to an insulin receptor on the surface of the cell. Once the insulin and the receptor are coupled, then the glucose may enter the cell.
If you have diabetes, it means either you don't make enough insulin, or the insulin you make simply doesn't work effectively.