There are a lot of misconceptions on Social Security so I'm surprised there isn't a sticky/FAQ (unless I missed it) on this topic at an ER forum, though probably no one wants to be responsible for the content...
Here's my candidate for the SS Trust Fund Sticky FAQ:
First: The blind men who described an elephant all had different stories, depending on which part they felt. One said it was like a snake, another a rope, another a tree, another a wall. Describing the SS trust fund depends on your perspective.
Accounting: The TF is just a spreadsheet that records each period's revenue and expenditures and accumulates the difference with interest.
Political Agreement: The TF keeps track of a compromise between people who wanted a bigger SS program and people who wanted a smaller program. They agreed that SS benefits would be fully funded by a dedicated tax, and that tax would not be used for anything else. The accounting worksheet tells us whether we're slipping off that agreement, and by how much.
Tax Burden: Since the SS tax is a regressive tax on labor income, while the FIT is a progressive tax on labor and capital income, different people are more impacted by one or the other. When the worksheet shows that SS is out of balance, we know that the groups are bearing more or less of the total burden of government than agreed.
Legal: Under current law, the Treasury Secretary is both allowed and obligated to pay SS benefits as long as the worksheet shows a positive balance. When the balance gets too close to zero, s/he will have to defer benefits until tax revenue refills the bucket enough to send the next batch of checks, and then s/he will send them.
Political Decisions: Because of the Legal above, gridlock in Congress means that SS benefits will be paid in full as scheduled until the worksheet gets too small. Then they will be paid at a reduced annual rate. Changing laws requires two houses of Congress plus the Prez, and they seem to be pretty polarized. Therefore the "full benefits till the big step down" scenario could actually occur.
Macro-economic: The federal gov't impact on the macro-economy depends only on total spending and total taxes. Any internal government accounting is irrelevant.
My Taxes: It depends on my personal tax situation. Is the SS tax a bigger deal to me than the FIT? or is FIT bigger? or are they about the same? In the third case, I really don't care where they get the money for SS. If one is noticeably bigger than the other, then how Congress deals with the TF imbalance can impact my tax bill.