The "biscuit" is the card the president is meant to keep close at hand. It is a card bearing nuclear launch codes that the president has to have in order to launch a nuclear attack. The president and his security staff is never, ever supposed to lose the biscuit.
General Hugh Shelton, who served under President Clinton as Chairman of the Joint Chiefs of Staff, wrote about the loss of the biscuit (nuclear launch code card) in his memoir book titled Without Hesitation: The Odyssey of an American Warrior. At one point during the Clinton administration, Shelton stated that the "biscuit" with its codes were actually missing for months. He called this a very big deal; it was a gargantuan deal.