Please explain what this Social security data means, & how is the cash flow surplus deficit calculated?

Appendix A. 2021 Trust Fund Operations Source: CRS, based on SSA, OCACT, "Financial Data for a Selected Time Period," hetps://wmw.ssa.govioact/ ProgData/allOps.html. Notes: OASI = Old-Age and Survivors Insurance. DI = Disability Insurance. OASDI = Old. Age, Survivors, and Disability Insurance. The Combined OASDI Trust Fund values may not necessarily equal the sum of its rounded components. Appendix A. 2021 Trust Fund Operations Source: CRS, based on SSA, OCACT, "Financial Data for a Selected Time Period," hetps://wmw.ssa.govioact/ ProgData/allOps.html. Notes: OASI = Old-Age and Survivors Insurance. DI = Disability Insurance. OASDI = Old. Age, Survivors, and Disability Insurance. The Combined OASDI Trust Fund values may not necessarily equal the sum of its rounded components