A lender will look at your credit and work history, income, and assets to determine the maximum purchase price/loan amount you are qualified for based on current market interest rates. Once you get pre-approved, you’ll receive a letter stating how much you are qualified for (based on the information you provided).
It is important to remember that the amount you are qualified for can change if your circumstances change or if interest rates change. A mortgage pre-approval is very helpful when establishing your budget, and it makes you more attractive to sellers. If you are searching for new home, it makes sense to get pre-approved and provide your Real Estate Agent with your pre-approval letter so they can start showing you homes.