SS is a pay as you go system. Currently, it is paying out more in benefits than it is taking in in revenue. The shortfall is being made up by redeeming the non-market, interest bearing T-bills in the SS Trust Fund, which contains $2.6 trillion in what amounts to IOUs.
The SSTF is included in the $16 trillion national debt and held under "Intra-governmental Holdings." In reality, the SSTF just represents the good faith and credit of the USG to honor the T-bills. Once the T-bills run out, the SSA can only pay out in benefits what it receives in revenue, which will reduce benefits across the board.
When SS was running a "surplus," more revenue than the amount of benefits paid, the "surplus" was deposited into the General Fund for use elsewhere and Treasury issued non-market T-bills in the amount of the "surplus" and deposited them into the SSTF.