A lender's conditional commitment to loan you a specific amount, based on a review of your finances.
A mortgage pre-approval is a lender's written estimate of how much they will lend you after reviewing your income, debts, and credit. It is stronger than a pre-qualification and signals to sellers that you are a serious, credible buyer. The rate-shopping inquiries behind it count as a single hard inquiry if done within a short window.
A pre-approval letter strengthens your offer in a competitive housing market.