To roar is to make a ferociously loud sound, like a lion or a tiger. If you go to a football game, you'll hear the crowd roar whenever their team scores a goal.
A wild animal roars when it makes its long, full cry, and a person (or a thing) can emit a similar roar. Your boisterous uncle roars with laughter, and the frustrated substitute teacher might roar angrily at the class. Motorcycles sometimes roar down the street, and a judge might silence a courtroom when she roars, "Order!" Roar can also act as a noun, meaning the sound itself — like the roar of the crowd that drowns out your voice. Roar comes from an imitative Old English root.