Pre-approval means that the bank has reviewed written evidence of your income, assets, expenses, credit and liabilities and has agreed in a time-limited agreement to fund your mortgage. Pre-approval is much preferred to pre-qualification because it allows you to shop for a home as if you were a cash buyer. It also gives you the ability to negotiate a sale price, especially if the seller cannot find another pre-approved or cash buyer for the home.

Keller Mortgage is a great place to start.