Lorelei is a name given to a legendary creature in German myth. The story goes that a beautiful siren named Lorelei would sit atop a rock on the side of the Rhine River and sing a song that would mesmerize sailors passing by. The singing would distract the sailors, causing them to crash their ships on the rocks and sink to the bottom of the river.
Think of it like a mermaid who sings but instead of saving sailors, she causes their demise. People talk about Lorelei when they refer to the power of music or when they want to warn others about dangerous distractions. It's just a mythical story, so don't be afraid of swimming in the river or listening to beautiful songs.