Original: Before Persephone was released to Hermes, who had been sent to retrieve her, Hades tricked her into eating pomegranate seeds, (six or three according to the telling) which forced her to return to the underworld for a period each year.
Simplified: When Demeter went to the Underworld to rescue her Persephone, Hades forced Persephone to eat the pomegranate. After she ate this fruit it was supposed to keep her in the underworld with Hades so she would be forced to marry him.