One you have determined how much you will spend on your new home, which loan options you would like to apply for and what mortgage rate you prefer, you can send your information to the lender of your choice and see how much you can be pre-approved for. Your pre-approval will also detail other terms of your loan, such as your interest rate and the length of time you have to pay the money back.
If you play your cards right and meet all the requirements for the loan of your choice (i.e. have a proper credit score and enough income to back your purchase, etc.) you may get pre-approved for the loan amount you requested or even more. If you are pre-approved, you can finally start looking for homes seriously, as you will be able to determine exactly how much you can afford.
With your pre-approved loan amount in mind, you can start putting offers on homes you are interested in and offer the down payment listed in your loan pre-approval agreement.
If your offer is accepted, you can put in your application for the actual loan form the lender that pre-approved you.
The lender will have to honor the amount they pre-approved you for.
However, if something happens and your offer is not accepted, you will not need to enter into the loan agreement.
You will only need to enter your pre-approved loan agreement if your offer is accepted and you must pay the seller for the property.