On some level, Henry W. Longfellow knew that he was no expert when it came to the legends of Native American peoples. But the dude wanted to write a poem about these legends either way, so here's what he did to protect himself: He created a short introduction that explained how he once heard a Native American singer and storyteller named Nawadaha sing a song about a great hero named Hiawatha.
The truth is that all of this is totally made up and Longfellow created the story of Hiawatha by mashing together a bunch of stories from totally unrelated traditions. But this intro has a way of making everything seem more legit and accurate than it actually is. That's why instead of "Hiawatha" we get "The Song of Hiawatha." Longfellow knew that there would be people out there who'd call him out on his inaccuracies sooner or later, so he inserted this intro and called the whole poem a "song" as if to say, "Don't blame me. Blame Nawadaha."