SSDI stands for Social Security Disability Insurance. SSDI is described as insurance because those who qualify have been working long enough to actually make contributions to the Social Security trust fund through their payment of FICA Social Security taxes. FICA is a payroll tax automatically deducted from your paycheck. To qualify for SSDI, you must suffer from a disability, be younger than 65, and have earned enough "work credits". Read more about SSDI vs. SSDI or how to qualify for SSDI.