Morgage Pre-Approval means that a Lender has verified income, outstanding credit balances, and has looked at a credit report in about the same details as if you had already applied for the morgage loan.
With a Pre-Approved Mortgage, you know where you stand as a borrower. You have a clear picture of the type of mortgage, the interest rate and what your mortgage payments will be. Plus, most Lenders will give you a rate-lock for 60 or 90 days after the Pre-Approval Date, so you're not caught if market rates change while you're home hunting.
When you buy and submit the mortgage application, the Lender will re-verify assets, income and debt to ensure nothing has changed since the Pre-Approval. The property will be appraised to ensure it meets Lender and Lending Ratio Guidelines. And it's important to note that even when you're pre-approved, the property must also be approved by the Lender, before you get a "Mortgage Commitment."