Amortization
The schedule by which an installment loan's balance is paid down over time.
Each payment on an amortizing loan splits between interest (computed on the remaining balance) and principal (which actually reduces the balance). Early payments are interest-heavy; later payments are principal-heavy.
On a 30-year mortgage, roughly the first third of the schedule is mostly interest. Extra principal payments early in the life of the loan have outsized leverage on the total interest you pay.
Calcly's amortization tools render the full month-by-month schedule and let you simulate extra payments to see the break-even.