A pre-qualification is an estimate that a lender will make based on basic financial information like your credit score, income, and debt load. It gives you a general idea of how much you qualify for and the rate you can expect.
A pre-approval is a more accurate version of the pre-qualification, based on extensive financial documentation. You will be given a written commitment with a locked-in interest rate for up to 120 days.
Having a pre-approval letter in hand will show sellers that you're a serious buyer, which can help in bidding wars. Real estate agents will likely also prefer that you be at least pre-qualified, so they don't waste time showing you homes outside of your affordability.
You don't need to make a final choice on the lender you'll work with at this stage. You can get pre-qualified through multiple lenders to see where you can get the best rate, and make your final choice once you're closer to buying. Different lenders can offer different options and rates, so research is important. Your real estate agent will be able to recommend lenders you might like to talk to based on your specific needs.