Mortgages usually include some prepayment privileges which give you the right to pay some extra money towards your principal and thus reduce the amount of interest you’ll pay in the future.
For example, some lenders allow prepayment of up to 20% of the original mortgage balance per year through lump sum payments (can happen any time throughout the year) or you can increase the regular scheduled payments by up to 20% (the extra going straight to your principal).
These privileges are different for each lender and sometimes for different mortgage products, so you always want to ask for an explanation of the mortgage prepayment privileges when you are comparing different mortgage options. Often mortgage products that have the lowest rates will have reduced privileges as a tradeoff.