Longfellow introduces this poem by saying that, if you asked him where his story comes from, he'd answer by saying that the story comes from the forest and the rivers and from nature itself. In other words, he's suggesting that he's about to tell us a legend that comes from nature just as much as it comes from the mouth of any one person.
More specifically, Longfellow says that the story he's about to tell comes from a singer named Nawadaha, who himself found them in the forests and brooks, etc. This dude starts singing a song about a guy named Hiawatha.
And, if the title of Longfellow's poem is any indication, we can guess that this musician is about to sing about the main character of this story.
You might notice that this story talks about how Hiawatha went through great suffering in order to "advance his people!" (I.78). Sounds a lot like the story of Christ, eh? Well, you'll want to watch out for stuff like this, since Longfellow is pretty blatant about applying his own Christian faith to the faith of Native Americans.