From Wikipedia: Social Security Trust Fund

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The Federal Old-Age and Survivors Insurance Trust Fund and Federal Disability Insurance Trust Fund (collectively, the Social Security Trust Fund or Trust Funds) are trust funds that provide for payment of Social Security (Old-Age, Survivors, and Disability Insurance; OASDI) benefits administered by the United States Social Security Administration.[1][2][3]

The Social Security Administration collects payroll taxes and uses the money collected to pay Old-Age, Survivors, and Disability Insurance benefits by way of trust funds. When the program runs a surplus, the excess funds increase the value of the Trust Fund. As of 2021, the Trust Fund contained (or alternatively, was owed) $2.908 trillion [4] The Trust Fund is required by law to be invested in non-marketable securities issued and guaranteed by the "full faith and credit" of the federal government. These securities earn a market rate of interest.[5]

Excess funds are used by the government for non-Social Security purposes, creating the obligations to the Social Security Administration and thus program recipients. However, Congress could cut these obligations by altering the law. Trust Fund obligations are considered "intra-governmental" debt, a component of the "public" or "national" debt. As of June 2015, the intragovernmental debt was $5.1 trillion of the $18.2 trillion national debt.