What it is: The debt ceiling is a legal cap on the amount of money the Treasury can borrow to fund existing government functions. It essentially authorizes the Treasury to borrow the money necessary to pay the bills incurred by the federal government.
Where it came from: Before 1917, Congress authorized the Treasury to issue bonds for specific purposes. But that meant approving every bond separately. To fund World War I, Congress decided to give the Treasury more latitude by instituting caps on how much it could borrow through each type of bond, rather than forcing it to get every new bond approved separately. In 1939, this was changed so that most bonds were bound by the same limit, effectively creating the general debt ceiling we have today.