Amortization is the way a fixed-payment loan is paid down over time. Each payment is the same size, but what it buys changes. In the beginning, most of the payment covers interest on the large remaining balance and only a little reduces the balance. As the balance shrinks, the interest portion shrinks with it, and more of each payment goes to principal. The schedule that lists this month by month is an amortization table.
Example: a $400,000 mortgage at 6.5% over 30 years has a payment of about $2,528. In the first month, $2,167 is interest and $361 is principal. In year 10, the split is roughly $1,760 interest and $770 principal. In year 25, about $640 interest and $1,890 principal. Over the full 30 years, total interest comes to about $510,000, more than the amount borrowed.
The shape explains a few things. Selling in the first few years builds almost no equity from payments, which is why closing costs weigh so heavily on short stays. Extra principal payments early in the loan skip the most interest-heavy part of the schedule; an extra $200 a month from the start on the loan above cuts about six years and $120,000 of interest.
The same math runs car loans, student loans, and personal loans on shorter timelines.
The common mistake is refinancing repeatedly into a fresh 30-year loan, which resets the schedule to the interest-heavy start each time. Refinancing into a shorter term, or keeping the old payment amount, avoids that.