We know from the title that our speaker is Demeter, goddess of agriculture, mother to Persephone and wife to Zeus. We also know, after spending a moment or two in the poem, that she isn't too happy.
So what's all the hubbub about? You see, Demeter's angry that Persephone is down in the underworld, and she is ready to have some words with Hades—Persephone's uncle who took her down there to be his wife. She wants him to realize what a horrible thing he's done; she wants him to get it. But she also spends much of the poem concerned with humanity's tendencies towards selfishness in general, and what the repercussions of selfish actions might involve.
In her mind, then, the worst thing that someone can do is act on their desires without thinking. And that's exactly what Hades did… with her daughter. Naturally, she's pretty upset—vengeful, even. Those Greek gods could get pretty intense, you know.
So yeah, those repercussions we talked about? They're nothing good, unless you don't mind going hungry. We guess it's never a good idea to anger an agriculture god or goddess.