The myth of Persephone, Demeter, and Hades takes place in two locations that couldn't be more different: the land of the living and the land of the dead.
Demeter is the goddess of agriculture, so she's closely tied to the land of the living. In fact, it's Demeter that allows the crops to grow that keep humans alive. Hades, on the other hand, is the king of the dead. He rules the underworld, the place where all souls go when they die. Since these gods are like polar opposites, it's no wonder Demeter is so p.o.'ed when her daughter is married off to Hades and dragged underground.
In this myth, the world of the living isn't all flowers, strawberries, and sunshine, though. When Persephone is with Hades, Demeter doesn't allow anything to grow – it might as well be the underworld because of the plants are dead and humans struggle to feed themselves. During the part of the year that Persephone gets to live with her mom, though, Demeter brings the earth back to life.