Preaching; pontificating; voicing one's opinion loudly in the hopes it will gain supporters. Comes from a time when wooden boxes were used to ship soapflakes in, and were then put into use as pulpits for street preachers.
"Shirley, quit your soapboxing and give THIS guy's opinion a thought or two."
The act of lecturing, spouting, ranting about a particular subject to attempt to make a point or a stand. Derives from the days when people, such as ministers, actors, strikers, abolitionists, rights defenders, etc. would take a sturdy soapbox (soap came in wooden crates then), carry it to a wtreet corner, and begin preaching their point to the passerbyers.
That point of conversation led her to soapbox about the subject until she knew she made her point.