APR is the Annual Percentage Rate of a mortgage. Unlike the interest rate (which only reflects the cost of the money you borrow), APR also includes most of the upfront fees you pay to get the loan: origination fees, discount points, mortgage insurance premiums, and certain closing costs.
Because APR includes those fees, two loans with the exact same interest rate can have very different APRs. A loan with a 6.5% rate and a 1-point origination fee will have a higher APR than a loan with a 6.5% rate and no points. Comparing APRs is the cleanest way to compare two real loan offers side by side.
APR is required to be disclosed on every consumer mortgage by the federal Truth in Lending Act. You'll see it on the Loan Estimate and the Closing Disclosure. If a lender quotes you only an interest rate without an APR, ask for both — that's how you compare apples to apples.