Great-grandmother Spaulding, who is ninety years old,decides it's time to die, and declares it to her family.
They all gather around her bed at the boarding house, and she calls them to her one at a time. When she calls Douglas, he's crying, of course, because he's Douglas and she's just told him she's dying.
She tells him to cut it out, saying that his cells die and regenerate themselves every seven years, and he doesn't cry when he cuts his nails, does he?
Which is one way to look at it.
When the whole family is gathered around again, she makes them all promise to do specific chores the next day. She says they shouldn't throw any "Halloween parties" to mourn her, then tells them all to leave her alone.
She curls up under her covers and goes to sleep for the final time, listening to the family moving in the house below. Bradbury's way of saying it is, "And the sea moved her back down to the shore."
Pretty nice imagery, in our humble Shmoopy opinion.