A Federal agency is only allowed to spend money if Congress provides the authority by law for that spending. That permission to spend is called “budget authority.”
Budget authority can be granted through an appropriation law, which specifies a purpose, usually a maximum amount of money, and a set time period. Budget authority can also be granted for spending unused funds from a previous year, or to spend money that the agency takes in (e.g., the National Park Service is authorized to spend fees collected for park admission regardless of the amount).
The total amount of all obligation budget authority including unobligated balances carried forward, adjustments to unobligated balances carried forward, appropriated amounts, and other budgetary resources, as of the reported date.