The pre-approval process is the first step in buying a property. Most sellers appreciate seeing a pre-approval letter to know that a buyer is serious about making an offer. This stage involves turning over financial documentation such as your credit report, employment history (self-employed buyers might need additional documentation), debt-to-income ratio, etc., to a lender so they can approve you for a loan. From there, you can shop for mortgage rates that work best within your budget.
You might also need a bridge loan to "bridge" the gap between a transitional period where you're moving from one house to another. Bridge loans supply you with money on hand to purchase a home. They do not replace mortgage rates. In short, the pre-approval process allows you to establish your overall budget for buying a house.