A soapbox is anything a public speaker stands on to address a crowd. A first grader giving a speech on the playground in favor of longer recess might use her lunchbox as a soapbox.
The original soapboxes were just that — wooden boxes originally holding bars of soap (in the days before cardboard). Just before World War I, street speeches were common, and speakers often used soapboxes to raise themselves above their audience. Since then, "on a soapbox" has become a metaphor for "expressing very strong opinions about a topic." If you lecture your meat-loving family about vegetarianism again, they might tell you to shut up and get off your soapbox.