How amortization works
A fixed-rate loan has one level payment, worked out so the balance reaches zero on the last payment:
M = P × r ÷ (1 − (1 + r)−n)
P is the amount borrowed, r the yearly rate divided by 12 and n the number of monthly payments. Each month the lender charges interest on the balance (balance × r); whatever is left of the payment repays principal. Early on the balance is large, so interest takes a big share; as it shrinks, more of the same payment goes to principal, which is why the columns in the chart change colour over time.
Worked example: $25,000 at 6.5% for 5 years. r = 0.065 ÷ 12 = 0.0054167 and n = 60, so M = $489.15. The first month's interest is $25,000 × 0.0054167 = $135.42, leaving $353.73 for principal. Over 60 payments you pay $4,349.20 of interest.
Payments are rounded to the cent, so the last one is adjusted by a few cents to land exactly on zero. Switch the table to "By year" for calendar-year totals, which is handy for comparing against a year-end statement.