Vanity metrics are numbers that go up and to the right without telling you whether the business is actually working. Cumulative downloads, page views, sign-ups, and 'reach' are the canonical examples, they grow even when retention, monetization, or unit economics are weak.
The antidote is actionable metrics tied to user behavior and revenue: activation, retention, payback period, and expansion. Investors are trained to spot vanity metrics, and decks that lean on them tend to lose credibility on the Traction slide.