Matt Y. explains the trust fund simply enough that anyone can understand:
Social Security, similarly, has been saving money for the past 20 years and will continue to do so for the next 14. At that point, it will slowly begin paying down its savings. This is not a Day of Reckoning for Social Security. At best, it’s a Day of Reckoning for the General Fund that owes Social Security the money.
Some people of my acquaintance believe that this is a distinction without a difference, since the General Fund is the government and the Social Security Trust Fund is just a different bit of the government. But the two notional entities have different revenue streams — the SSTF is mostly financed by payroll taxes on the middle class while the GF is mostly financed by income taxes on high earners. When the GF borrowed from the SSTF America’s middle class was lending money to America’s rich with an understanding that the money would be repaid with interest. Similarly, I lent money to the Riggs Bank with the assumption that my money would be repaid with a (very modest amount of) interest. If the Riggs Bank proposed to just not pay me back, they’d be in trouble. Today, the agents of America’s rich in the Republican Party are proposing that the wealthy default on their debt to the middle class. That this proposal exists is the only thing that Social Security must “reckon” with for the next several decades.
I’d add that if you really believe the federal government will default on its debt obligations, whether held by the SSTF, the Chinese, or US citizens, then you might also want to consider stocking up on guns and gold.