“I’m going to do something unprecedented tonight,” Andrew Yang said during Thursday's Democratic presidential debate, announcing in his opening remarks that he would give out $120,000 in total to 10 randomly selected families watching. Mr. Yang has staked his candidacy on the idea of giving every American citizen $1,000 a month, no questions asked, to end poverty, protect workers from the threat of technological disruption and usher in a new era of “human capitalism.” How much do you know about universal basic income, or U.B.I.?