A mortgage pre-approval is a written commitment from a lender stating how much they're willing to lend you, based on a full review of your income, assets, credit, and debts. It's stronger than a pre-qualification, which is just a rough estimate.
Sellers and real estate agents take pre-approved buyers far more seriously. In competitive markets, a pre-approval letter is essentially required to make an offer. It tells the seller you can actually close.
Pre-approvals typically expire in 60–90 days, since credit reports and rates change. If you don't find a home in time, you can usually refresh the pre-approval with updated documents.