Amortization is the process of paying off a loan through a schedule of payments, each split between interest owed and the principal balance itself.
The split is not constant. Early payments are mostly interest and barely reduce the balance; later payments are mostly principal. This is why a mortgage balance seems to move so slowly in the first years, and why an extra payment made early removes far more total interest than the same payment made late — it reduces the balance that all future interest is calculated on.