An amortization schedule shows what each repayment is actually doing - how much clears interest and how much reduces what you owe. Early on, almost all of it is interest, which is why the balance barely moves for years.
It is the month-by-month breakdown of a loan: for each repayment, how much goes to interest, how much reduces the balance, and what is left owing. The repayment stays the same, but the split between the two changes every single month.
Because interest is charged on what you still owe, and at the start you owe almost everything. On a $500,000 loan at 6%, the first repayment of about $2,998 is roughly $2,500 interest and only $498 principal - so the balance drops by less than a fifth of what you paid.
That reverses over time. By the final year, almost all of each repayment is principal.
The point where principal first exceeds interest in a single repayment is later than most people expect - typically well past the halfway mark of a 30-year loan at ordinary rates. The schedule below shows exactly when it happens for your numbers.
Because every extra dollar comes off the balance that interest is charged on for the entire rest of the loan. The same dollar paid in the final year saves almost nothing. The loan and mortgage calculator models extra repayments and offset balances together.