Private mortgage insurance (PMI) is a premium a lender adds when your loan-to-value is high, typically above 80%, which means a down payment under about 20%. It protects the lender against loss if you default; it does not protect you or your equity. The cost is bundled into your monthly payment until you build enough equity to drop it.
PMI generally applies to owner-occupied conventional loans; pure investment-property financing is usually structured to avoid it by requiring a larger down payment instead. For a house-hacker buying a small multifamily to live in, though, a low-down-payment loan carrying PMI can be the cheapest way in. Because PMI is tied to loan-to-value, paying the balance down or rising value can eventually let you request its removal.