Literally, a soapbox is a wooden box. Long ago in America, shops got their soap delivered in big wooden boxes, and the empty boxes were everywhere. If you wanted to speak to people in the street — about politics, about God, about anything you cared about — you turned one over, stood on it, and started talking. That is the whole picture behind the word.

Today people almost always use it the second way. If someone 'is on their soapbox', they are going on and on about an opinion they feel strongly about, loudly, and usually without much interest in what anyone else thinks. You can 'get on your soapbox' (start) and 'get off your soapbox' (stop). The word also works as a noun for the topic itself: 'Recycling is her soapbox' means that is the thing she always brings up.

The feeling is gently negative, but it is a soft, laughing kind of negative. It is what you say about a friend or an uncle, not what you say about an enemy. You can even use it about yourself, and English speakers often do — 'Sorry, I'll get off my soapbox now' is a polite, slightly joking way to stop yourself when you notice you have been talking too long. That self-use is very common, and it is a nice thing for a learner to have.

There is one more, quite separate use in British English: a 'soapbox' or 'soapbox cart' is a homemade toy car with no engine that children race down a hill. If you hear it with 'race' or 'derby', that is this meaning, not the speech one.