In a loan with a fixed payment, each payment is split between interest and paying down the debt. Each month's interest is worked out on the outstanding balance, which is largest at the beginning.
An example with numbers
For $10,000 at 6% nominal a year, the first month's interest is $50 if the monthly rate is 0.5%. With a $300 payment, $250 goes to principal and $9,750 is left. The next month, the interest would be $48.75.
Try it with your numbers
Open your loan's amortization schedule and compare the first month with one near the end. See how the split changes while the payment stays the same.
Example figures for learning purposes. They are not a recommendation and do not replace professional advice.