Demeter was the Goddess of corn, grain& the harvest. She was the daughter of Kronos&Rhea. It was believed that Demeter made the crops grow each year; thus the first loaf of bread made from the annual harvest was offered to her. She was the goddess of the earth, of agriculture, and of fertility in general. Sacred to her are livestock and agricultural products, poppy, narcissus and the crane. 

Demeter was intimately associated with the seasons. Her daughter Persephone was abducted by Hades to be his wife in the Underworld. Demeter laid a curse on the world that made the plants to wither ans die, and the land to become desolate.  Zeus, alarmed for the barren earth, sought for Persephone's return. However, because she has eaten while in the underworld, Hades had a claim on her. Therefore, it was decreed that Persephone would spend four months each year in the Underworld and the other eight months in the living world. 

Demeter was also known for founding the Eleusinian Mysteries. They were initiations held every year for the cult  of Demeter&Persephone based at Eleusis in Ancient Greece. They are the "most famous of the secret religious rites of ancient Greece". These were huge festivals and held every five years and very important events for many centuries, Yet, little is known about them as those attending were sworn to secrecy. It is thought that that central tenet around which the mysteries revolved was that just like grain returns every spring after its harvest and the winter lull, so does the human soul after the death of the body, reincarnated in a next life.