Rate drives the payment
Your interest rate is the number used to calculate principal and interest each month. It is the figure that determines the payment you live with.
APR is a disclosure. It takes the rate and adds certain financing costs, then expresses the whole thing as an annualized percentage.
When APR helps and when it misleads
APR is useful for comparing two offers with similar structures. A meaningfully higher APR next to the same rate usually signals higher lender fees or points.
It misleads when you will not keep the loan long. APR assumes you hold to term. If you plan to sell or refinance in five years, compare five year total cost instead: payments plus upfront costs, minus remaining balance difference.