With a repayment mortgage, or capital repayment mortgage, to give it its full name, you pay back part of the mortgage capital and interest each month. At the outset, most of your monthly payments will comprise of interest; over time, more of your monthly payment will be repaying the capital.
With a repayment mortgage, you are guaranteed to repay the full mortgage by the end of your mortgage term, provided you make your repayments in full each month.
As a mortgage is secured against your home, it could be repossessed if you do not keep up the mortgage repayments.