These are the dangerous ladies who lure men to their deaths with their voices. Odysseus becomes the first mortal to live to tell the tale, because he has his men tie him to the mast while they plug their ears and sail on by. This is what they say:
Come this way, honored Odysseus, great glory of the Achaians, and stay your ship, so that you can listen here to our singing; for no one else has ever sailed past this place in his black ship until he has listened to the honey-sweet voice that issues from our lips; then goes on, well-pleased, knowing more than ever he did; for we know everything that the Argives and Trojans did and suffered in wide Troy through the gods' despite. Over all the generous earth we know everything that happens. (12.184-196)
What's cool is that people usually think of the Sirens as being super sexy and luring men by their feminine charms—but here, it sounds like they're actually promising Odysseus knowledge.