that is how much the US gov't is expected to have to pay out in future costs for medicare and social security and doesn't have. aka the real national debt
for reference, that is $400k per household.
here is a video on the subject:
the presenter is obnoxious, but stick with it: the subject will play a large role in defining our global future.
the average american today is already overburdened with (mostly self-inflicted) debt and having difficulty making ends meet. add higher taxes to that to try to address the future cost problems and these people will have serious financial problems and be forced to reduce their consumption. considering that the US economy is 70% consumption, if they have problems, we all have problems.
add to this peak oil and climate change (which will each add costs and reduce productivity universally) and the next few decades start to look pretty bad for humanity and and downright horrific for the US.