An amortizing loan — most mortgages, auto loans, and standard business term loans — is structured so every payment is the same dollar amount for the life of the loan, but the mix inside each payment changes over time. Early payments are mostly interest, because the outstanding balance is still large; later payments are mostly principal, because the balance has shrunk.
An amortization schedule is a table showing, payment by payment, exactly how much went to interest versus principal, and what the remaining balance is afterward. It's useful for understanding total interest cost, and for deciding whether an extra principal payment is worth making — it almost always is, early in the loan, when the interest share is highest.
The monthly payment itself is calculated once, up front, from the loan amount, interest rate, and term — it doesn't need to be recalculated each month unless the rate changes, as with an adjustable-rate loan.