Private mortgage insurance (PMI)
Private mortgage insurance (PMI) protects the lender, not you, if you stop paying your mortgage. Lenders generally require it when your down payment is less than 20 percent of the price.
New Jersey example
Say you buy a $400,000 home in Union County with 10% down (made-up numbers). Your $360,000 loan would generally need PMI. Under federal law, PMI on most loans made on or after July 29, 1999 ends automatically once the loan is paid down to 78% of the home's original value, which here is $312,000.
Official source
Everything You Wanted to Know About Buying a Home (NJ Department of Banking and Insurance)
This is a plain-English summary. Rules and dollar amounts change, so check the official source or a professional before you file or sign anything.