Obama has been saying that he will default on the obligations of the United States (treasury bonds) if Congress doesn't raise the federal debt limit.
That's scare mongering. Tax revenue is enough to cover two thirds of the federal spending. So pay off the bonds with tax revenue, pay the armed services (you're gonna need 'em). Pay social security 'cause of the political uproar that will result if you don't. Furlough all the civil servants, except maybe air traffic controllers. Stop buying (or at least paying for) new toys for the armed services. Stop highway construction, farm subsidies, Congressional pay and benefits. Close down Agriculture, Health & Human Services, Transportation, Commerce, CIA, NSA, EPA, FDA. If you run out of money, pay suppliers (including medical suppliers) with IOU's. We can make it. We might even find we can save serious money.