When you pay off your existing debt (mortgage) and create a new one, it is said that you're refinancing your mortgage. A borrower can do so for many reasons, including an interest rate drop, an improvement in his credit history, or his intention to switch his mortgage type into a different one. A mortgage refinance with no closing costs is one in which these costs are already included in your loan. With a mortgage refinance with no closing costs, you're not required to pay when refinancing.
A mortgage refinance with no closing costs typically includes settlement costs and fees such as underwriting and appraisal. However, some lenders may resort to certain tricks when offering you a mortgage refinance with no closing costs.