Prometheus is a figure from Greek mythology [stories from ancient Greece]. He was a Titan [a powerful god-like being] who stole fire from the gods and gave it to humans. Additionally, Prometheus is also known as the creator of humanity. As a punishment, Zeus [the king of the gods] had Prometheus chained to a rock where an eagle would eat his liver every day.
Very Simple Definition:
Prometheus is a character from Greek Mythology (old stories from Greece) who created the first humans, and brought them fire from the gods.