This form allows you to compare what would happen if you took one of two choices with a big chunk of cash you have -- paying off your mortgage, or investing it instead. This tries to take into account your tax situation and assumes you always itemize (even late into your mortgage when your interest will be lower -- I am assuming you give to charities, and pay state and local taxes too!!) Note that with the new tax cut your deductions may now be less valuable
Another way to think of this issue that I heard was to ask yourself this question: If you had your house fully paid off, would you take out a loan at today's rates to invest? If you answer no, just pay off the mortgage and sleep soundly at night. If you would, then keep your mortgage and invest the lump sum.