"Brace yourself" means to mentally prepare yourself to hear some exciting, shocking or disappointing news.
You've had many health problems recently, and have found a suspicious lump. After a biopsy, your doctor might come into your hospital room and say, "Brace yourself - the tumor was benign! You don't have cancer!"
You and your spouse are looking to buy your first house, and make a bid on one that is several thousand dollars below the asking price. Your real estate agent might phone you and say, "Brace yourself - the seller accepted your bid! You are now a homeowner!"