Piper is remembering or having a vision of hanging out with Jason on the roof of the Wilderness school and dancing with him.
Then she has a vision of being back in the department store, but instead of Medea being there, her mom—Aphrodite—is.
Aphrodite says Piper is good at sensing possibilities, and that the possible romance with Jason might happen.
She also explains why she didn't tell Piper's dad she was a god—he'd crack from the knowledge. She gives Piper a potion to erase his memories so he won't be permanently broken by knowing about gods and giants.
Aphrodite tells her that Gaea, the earth, is their enemy, which is supposed to be a big reveal, but mostly it's just another evil bad guy name. Anyway, Gaea is the earth, and she's tough.
The goddess adds something cryptic about bringing two sides together, and then the dream fades out.