The 'scarcity loop' is a behavioral loop that consists of three parts: opportunity, unpredictable rewards, and quick repeatability. In the context of slot machines, the opportunity is the chance to win money, the rewards are unpredictable as the outcome of each spin is random, and the quick repeatability allows players to immediately repeat the action, with the average player playing about 16 games per minute. This loop is highly effective at getting people hooked because it taps into deep-seated evolutionary behaviors related to resource acquisition and survival.