She said that the realm of Valinor, where the elves sail off to when they get "old" used to be part of the rest of Middle Earth. Apparently that's also where the Valar, who are effectively gods, live.
Middle Earth used to be falt back then, but some humans tried to sail to Valinor (stupid humans!) so the Valar decided to seperate the realms. The Valar changed the world into a ball, but left a straight path to Valinor that only the elves could use.
So you've got this round ball world with a straight path leading off to Valinor. Middle Earth is a cherry!! =)
Which makes me wonder, do the Valar have contests seeing who can tie the Middle Earth in a knot using just their tongue? =)