If you are asking about PMI (private mortgage insurance), than PMI is insurance payable to a lender or trustee for a pool of securities that may be required when taking out a mortgage loan. It is insurance to offset losses in the case where a mortgagor is not able to repay the loan and the lender is not able to recover its costs after foreclosure and sale of the mortgaged property. Typical rates are $55/mo. per $100,000 financed, or as high as $1,500/yr. for a typical $200,000 loan. The PMI may be payable up front, or it may be capitalized onto the loan in the case of single premium product.
This type of insurance is usually only required if the downpayment is less than 20% of the sales price or appraised value (in other words, if the loan-to-value ratio (LTV) is 80% or more). Once the principal is reduced to 80% of value, the PMI is often no longer required.