Prometheus is the figure in Greek mythology who stole fire from the gods and gave it to man. For this, he was chained to a rock, where every day a vulture was sent to gnaw at Prometheus's body.
The Prometheus figure thus represents defiance of the gods and the empowerment of men, though Prometheus himself is punished. In Mary Shelley's tale, Frankenstein does the very thing that was previously accomplished only by God: the creation of a living being. But the story takes place in a contemporaneous setting, so Frankenstein is a modern counterpart to the prototypical mythic figure who, while not "creating life," gave man a gift which enabled him to accomplish things never done before.
Mary Shelley wrote at a time when the Romantic movement was conceptualizing man as a semi-divine figure. The old limits imposed by religion upon man had begun to be rejected during the eighteenth-century Enlightenment, and during the Romantic period which followed, writers and people in the arts generally were committed to this changed view of humanity. The irony of Shelley's story is that man's new power to create backfires against him. It is a moral parable about the weaknesses of human beings in spite of their new ability to advance and create.