Amortization
Definition
Amortization is how each loan payment divides between interest and the balance itself. Early payments lean toward interest, and the share going to the balance grows as the loan runs.
On a fixed rate loan the payment stays the same while its composition changes every month. Interest is charged on the outstanding balance, so as that balance falls, less of each payment goes to interest and more to principal.
This is why a long loan feels slow at the start. Through the first years of a thirty year mortgage, most of each payment is interest.
Paying extra reduces the balance directly, which removes all the interest that would have been charged on it for the rest of the schedule.