This claim that is seen on TV commercials is really one of the more disreputable of the campaign claims being made these days.
Social Security benefits were set up to be paid from a trust fund, which is not literal but just a matter of bookkeeping, and "it" is funded by payroll taxes. Those moneys have not kept up with conditions and with Congress' frequent tinkering with the benefits structure.
What has happened is that the federal government, meaning Congress, has borrowed from that trust fund repeatedly and left IOUs behind.
As a result, it has been said for many years that the trust fund will soon be exhausted.
That's the trust fund. However, Congress will never allow beneficiaries to be cut off.
What will happen is that more money will be taken from general revenues in order to keep the benefits in force. Everyone in Congress knows this.
So for the Biden campaign to now contend that Trump wants to cut people off--or that the benefits will end in a few years because of a shortage in the trust fund--is terribly wrong. But people think that the elderly are easily terrified, so....
We have increased the national debt by 6 Trillion dollars in just the recent past, therefore what I've described will simply become another part of that story, whether or not the trust fund runs out or the payroll taxes are cut (which isn't likely).