From The National Center for Policy Analysis:

Generational accounting is a well-established methodology to measure the burden of government, says Laurence J. Kotlikoff, a professor of economics at Boston University and a senior fellow with the National Center for Policy Analysis. For example, a generational account for any given generation measures the generation's remaining lifetime net tax bill as a present value -- what the generation will pay net of what it will receive, all valued as of today. 

If the generational accounts of all current and future generations are added together, assuming no change in fiscal policy, the sum amounts to what all current and future citizens are going to pay, on net, in taxes to the government (measured as a present value).  This amount has to cover the government's official debt plus the present value of all future government purchases of goods and services (discretionary spending), explains Kotlikoff:  More here.