When you prequalify, the lender simply takes your word for the information you supply. When you are preapproved, the lender has verified this information.
Keep in mind, though, that even preapproval is not a guarantee of final loan acceptance, because interest rates may change. Higher interest rates mean higher payments, and that could mean you no longer qualify for the same loan amount.
Only when the lender commits to an interest rate (locks in the rate) can you be sure that you qualify for the loan.