A group of three men stand in a circle. They start to say "no soap radio" to each other, over and over, until the words lose all meaning. The phrase becomes an endless chant, and the men lose consciousness, chanting "no soap radio" until they starve to death on their feet.
This joke is funny because it's about the concept of a meaningless phrase that is repeated forever, the concept of the mind losing meaning in an action that is repeated forever. The joke ends exactly as it begins: the men are starving and chanting "no soap radio." The humor lies in the repetition and the absurdity of the men who starve forever while chanting a meaningless phrase.