The poem requires a basic understanding of a few characters from Greek mythology. Fortunately, Shmoop already has you covered. So first thing's first: go read all about Persephone, Demeter and Hades. Done? Great. If you're still fuzzy, we'll give you the gist: Persephone gets kidnapped by creepy Uncle Hades and Momma Demeter gets so mad she refuses to make any plants grow. Bummer.
Now for the poem. The speaker focuses our attention on the precise moment that Hades kidnaps Persephone. The poem begins with a girl (Persephone) off on her own picking flowers. She tugs and tugs at one particularly beautiful blossom and up pops Hades, who snatches her down to the underworld with him, which no one notices, because Persephone had "strayed." Uh oh.
Then the speaker addresses us, or at least reminds us of what the girl had always been told by Demeter: to be careful, to watch out for danger. The chilling last two lines explain why her mother had always been so overprotective—the world is a scary place and bad things can happen in the blink of an eye.