This may be an old expression but it still rings true today, especially in the current Real Estate market. You're ready to buy a new home. Your credit is good (at least Credit Karma tells you it is) and you have funds for a down payment. So you think it's time to start shopping. If you haven't contacted a mortgage lender and provided the necessary documents to get a pre-approval, then you have just "put the cart before the horse". 
Take Janie for example. She found the perfect house. But so did 3 other Buyers! She has been told that she can't submit an offer without a pre-approval, so she calls the lender and gathers her tax returns, pay stubs and other requested items. Too bad she wasn't prepared before she found that perfect house. Next thing she knows, the Seller has accepted an offer and the property is off the market. But when she finds the next she's prepared. And that pre-approval letter is good for up to 90 days! 
Don't be like Janie. Put the Horse in Front of the Cart :-)