A campaign budget is often regarded as a particular's campaign objective in terms of spend. Generally campaign budgets are expected not to be exceeded, which is why budgets are usually greater than or equal to the maximum spend.
Campaign budgets can be expressed as lifetime budgets, daily budgets, or even open budgets (unlimited) in which case other goals or objectives are pursued.
In AudioGO, the minimum campaign budget is $250 and is applied for the entire duration of the campaign. You can read more about how the budget is distributed across the campaign flight dates here.