soapbox
A platform, real or imagined, for giving strong public opinions.
A soapbox is a makeshift platform someone stands on to give a speech or express their opinions publicly, often passionately. The word comes from a time when people would literally stand on wooden crates (like the boxes soap was shipped in) to raise themselves above a crowd in a park or on a street corner so everyone could see and hear them.
Today, we mostly use soapbox as a metaphor. When someone gets on their soapbox, they start giving a long speech about something they feel strongly about, whether others asked to hear it or not. A student might get on their soapbox about longer recess, or a parent might get on their soapbox about the importance of reading.
There's also soapbox racing, a fun activity where kids build small, engineless cars (originally from wooden crates and wagon wheels) and race them downhill, steering but not pedaling. These gravity-powered racers still bear the name soapbox derby cars.
When someone tells you to “get off your soapbox,” they're suggesting you've been talking at people rather than talking with them.