Interest is the compensation a borrower pays to their lending institution in exchange for a loan. In a mortgage loan, interest is usually paid to the lender in regular payments along with repayment of the principal (the loan amount).
In fixed rate mortgages, the agreed-upon interest rate is locked in until the end of the mortgage term. Therefore, the mortgage payments will be constant every month under normal circumstances.
In variable rate mortgages, the interest rate will fluctuate according to the market, and the total mortgage payment is subject to change from month to month.