The interest rate is the price of borrowing money, expressed as a yearly percentage of your loan balance. It's the single biggest lever on what a mortgage actually costs you.
Here's the mechanic that surprises people: your monthly payment stays level on a fixed-rate loan, but each payment is split between interest and principal based on your remaining balance. Early on the balance is large, so most of the payment goes to interest; only over time does the split flip in your favor. That's why a seemingly small rate difference compounds into a huge difference in total interest over the life of the loan — try it yourself with the mortgage calculator.
Your rate depends on market conditions plus your own profile: credit score, down payment, loan type, and term. You can see where the market sits on the current mortgage rates page.
One caution: the interest rate is not the APR. The rate sets your monthly payment; APR bundles in fees to show the broader cost of the loan. When comparing lenders, look at both.