Trochaic Tetrameter
Traditional European literature usually follows an iambic pattern, where you have two syllables with the emphasis falling on the first. For example, "I got the bus and rode to school." But for Hiawatha, Longfellow decided to use the opposite kind of meter—something called trochaic—which puts the emphasis on every first syllable. The "tetrameter" part of the meter just means that there are four (tetra- means four) pairs of syllables in each line. For example, "Dead he lay there in the forest" (3.259).
Longfellow chose trochaic tetrameter because he felt like it more accurately represented the rhythms of Native American singing and chanting. Just think of a stereotypical chant like "Way ah Hey ah Wey ah Hey ah." That's the kind of sound Longfellow was going for, even though he was drawing really heavily on bad cultural stereotypes when he thought of it. The funny thing is that critics of Longfellow's time hated his use of trochaic tetrameter and couldn't even take it seriously. Some even accused of him plagiarism because Longfellow's meter was the exact same as that of an epic Finnish poem called the Kalevala that Longfellow had read and admired. People also found the meter so silly that they started writing their own poems with Longfellow's meter just to make fun of the guy. In case the rest of this module hasn't made it clear, Longfellow took flak from, well, pretty much everyone after writing this poem.