Soreness (DOMS, Delayed Onset Muscle Soreness) is a sign that the muscle has been damaged from an external load.
When the muscle is damaged by exercise, it must be repaired just as any other tissue. This process predominantly involves immune cells called macrophages and a few similar cells. They infiltrate the damaged tissue and begin to phagocytize the damaged cells. Essentially, this is the process of inflammation that you can see in a cut or burn on your skin. Additionally, macrophages have been shown to either release or be involved in the release of certain growth factors that aid in the healing of tissues. Also, the growth factors stimulate satellite cells to fuse with the damaged muscle cells; i.e., hypertrophy.
DOMS is a result of all this metabolic activity going on; the chemicals released by the macrophages during the process of phagocytosis stimulate the type IV receptors in the muscle, causing pain.
So, while you can't judge a workout's success by the presence or absence of DOMS, you can at least be aware that some growth is occuring.