Dictionary

A crate for packing soap.

Any physical or media platform which gives prominence to the person on it and the views they espouse.

Example: The madman obtained a soapbox which he stood on at the corner of Broadway and Wall street, to shout out his prophesy of the end of the world.

A talk about one's pet topic (or the topic itself), especially when only tangentially relevant to an ongoing discussion.

Example: He's been on his soapbox all day about the new football coach.

A soapbox car.

To give a speech from (or as if from) a soapbox.