They are either a women or winged creature whose singing puts any unwary sailor into a trance and lure them (on ships) on to rocks, so they could crash.
Odysseus warns his men about the Sirens and what they're about to encounter. To save Odysseus and make it out alive, they tied him (Odysseus) to a mast with beeswax in their ears. That's when the Sirens came around and start singing. The crew kept on rowing, while the Sirens dwindled away. After that, the crew peeled off the beeswax and untied Odysseus.