1. Sirens are mythical creatures, best known from the story of Odysseus/Ulysses in Greek mythology, where they tried to tempt sailors. When the sailors would get nearer, their ships would get wrecked on the rocks.
German mythology, too, has got its sirens, like, e.g., the "Lorelei" that lived in the river Rhein, and did pretty much the same thing as her Greek colleagues.
2. As a symbol, sirens therefore represents temptation, feminine seduction, and also deception.