"The Congress shall have power ... to borrow money on the credit of the United States." The Congress has the sole power to borrow money for the federal government. Thus, each year, the Congress must authorize the total amount of debt that the United States government may have.
One of the most decisive issues in Congress the last few years has been the questions on whether or not to extend the debt limit. The limit is the total amount that the United States is allowed to borrow. Since the US has been running a budget defict each year is is neccessary to periodically extend that debt limit to allow the US to continue operating the government.