A bond is a guarantee of future value via debt. In the case of government bonds, sold and managed by the Department of the Treasury or an individual department, citizens, businesses, or even foreign entities buy bonds from the government with a promise of return later on when the bond matures. The initial value of the bond purchased is called the "principal", the difference between the principal and the return is the interest rate.
Before the Naturalization Contingency Act was passed in 2015-I, only the Treasury could issue bonds. The law allowed the Department of Naturalization to issue its own bonds and exempted it from a government shutdown. In 2015-II, the government passed the LEAD Act which further decentralized departments and allowed then to issue bonds themselves. If a department fails to pay off its debt, the department may be taken over by the Department of the Treasury until all debts are paid.