Amortization is the process that turns a large loan into a series of equal payments that end at exactly zero. With a fixed-rate mortgage the payment never changes, but what each payment does changes every single month.
The mechanism
Each month, the interest you owe is the current balance times the monthly rate. Whatever is left of your fixed payment after covering that interest goes to principal. Because that principal payment lowers the balance, next month's interest is a little smaller, so a little more goes to principal. Repeat that a few hundred times and the balance curves down to zero.
The crossover point
Early on, when the balance is high, most of your payment is interest. Late in the loan, when the balance is small, most of it is principal. Somewhere in between is the crossover point, the first month where principal exceeds interest. On a 30-year loan at today's rates it usually lands somewhere around years 17 to 20, and it arrives sooner when the rate is lower. Our calculator marks it on the balance chart.
Why the start feels slow
Many buyers are surprised how little the balance moves in the first few years. That is not a trick, it is just math: interest is charged on a large balance, so the principal share is small at first. It is also why extra payments early in the loan are so powerful. A dollar of extra principal now removes interest from every remaining month.
Reading your schedule
An amortization schedule lists all of this, one row per payment. It tells you the balance at any point, the exact month PMI can come off, and how much interest you will have paid by a given year. It is worth a look before you sign.