Meaning

noun a crate for packing soap

noun a platform raised above the surrounding level to give prominence to the person on it

originFrom soap + box. The figurative use of the word derives from the fact that soap products in the US used to be commonly transported to stores in large wooden boxes. It used to be common for individuals to give impromptu rally speeches to their neighbors by finding something to stand on like a soapbox.

Etymology from Wiktionary, CC BY-SA 3.0