It is responsible for the production of melatonin, which has a role in circadian rhythm. Melatonin is a derivative of the amino acid tryptophan. The production of melatonin by the pineal gland is stimulated by darkness and inhibited by light.
The pineal gland is a midline structure and is often seen in plain skull X-rays as it is often calcified.
It is sometimes referred to as the "third eye" in occult religions, and is believed to be a dormant organ that can be awakened to enable communication with the astral plane.