When we examine any modern nation, it is at times necessary to borrow money in order to maintain or improve the functions of the government. The way the United States borrows money is different from the way an average person would borrow money. The United States government does not go to a bank, but instead issues debt through certificates such as bonds. These certificates are issued through the U.S. Treasury Department, a part of the executive branch. These certificates, often in the form of bonds, are sold to individuals, companies, organizations, and even foreign governments. This typically works on the promise that at a future date the United States will pay the value of the bond back with interest.
The Treasury Department, however, does not simply have the power to create and sell bonds as it pleases. It is, in fact, the legislative branch that has the power to borrow money on behalf of the U.S. government. There is a maximum amount of debt that the United States government can have and that is referred to as the debt ceiling. The Treasury Department cannot issue more debt than is permitted by the debt ceiling. If the Treasury Department wishes to issue more debt, then they are required to get congressional approval through a raising of the debt ceiling. Please find below an excerpt from Article 1 Section 8 of the Constitution which lays out the power of the legislative branch to borrow money on behalf of the U.S. government.
The Congress shall have Power To lay and collect Taxes, Duties, Imposts and Excises, to pay the Debts and provide for the common Defence and general Welfare of the United States; but all Duties, Imposts and Excises shall be uniform throughout the United States;
To borrow Money on the credit of the United States;
To regulate Commerce with foreign Nations, and among the several States, and with the Indian Tribes;