A pre-approval is a lender's conditional agreement to lend you up to a set amount. It tells you your budget before you make offers, and means you can move quickly, including at auction.
It usually lasts around 90 days, and it's conditional: the property still has to meet the lender's criteria, your circumstances need to stay the same, and the lender may want a valuation or a builder's report once you've found the home.
To get one, the lender assesses your income, living costs, debts, deposit and credit file, so this is where a credit check is done, with your consent. Applying to several lenders separately can leave several enquiries on your file, which is one reason to compare through an adviser first.
If your pre-approval is about to expire, it can usually be renewed with updated payslips and statements. Tell your adviser early if anything changes, such as a new job, a new debt or a different deposit.