Private mortgage insurance, PMI, is a cost you carry for someone else's protection. If you stop paying, the policy pays the lender, not you. The CFPB explains that it usually comes with a conventional loan when your down payment is under 20% of the price, and that it can be charged as a monthly premium added to your payment, a single premium at closing, or a mix of the two.
The monthly form is the common one, and its useful feature is that it ends. You can ask your servicer in writing to cancel it once the balance is scheduled to reach 80% of the home's original value, or sooner if extra payments get you there, provided your payment history is good and the home has not lost value. If you do nothing, the servicer must end it on the date the balance is scheduled to reach 78%, and in any case the month after the midpoint of the loan's term, as the CFPB sets out. Original value here means the price or appraisal when you bought, so a rise in house prices does not trigger the automatic stop.
Other countries handle a small deposit differently. Australian lenders charge a one-off lenders mortgage insurance premium, and Canada requires mortgage default insurance from a public or private insurer, in both cases usually added to the loan instead of billed monthly. Britain, Ireland and most of Europe have no borrower-paid equivalent and price the risk into the interest rate.
The mortgage calculator charges PMI on American loans as a yearly percentage of the loan, collected monthly, for as long as the balance is above 80% of the price. An extra monthly payment brings the stop date forward, and the result line tells you the month it ends.