There's a fine line between a rabble-rouser and a great public speaker. A rabble-rouser's highest priority is getting people worked up, especially by appealing to their prejudices and ignorance, with the ultimate goal of his own political support. It's a nineteenth century phrase that combines rouse, or "stir up," with rabble, which originally meant "a pack of animals," and later came to also mean "an unruly mob of people."