Social Security Disability Insurance (SSDI) is a federal insurance program that provides financial support to individuals who are unable to work due to a disability. This program is administered by the Social Security Administration (SSA) and is funded through payroll taxes paid by workers and their employers.

Eligibility for SSDI is determined by the SSA, and requires that individuals have a severe physical or mental impairment that is expected to last at least one year or result in death. In order to qualify for SSDI, individuals must also have a certain amount of "work credits" based on their earnings and the length of time they have been working.

Once an individual is approved for SSDI, they will receive monthly cash benefits to help cover their basic living expenses. The amount of these benefits is based on the individual's past earnings and is generally about 60-70% of their previous income.

In addition to cash benefits, individuals who receive SSDI may also be eligible for other forms of support, including Medicare and Medicaid. Medicare is a federal health insurance program that provides coverage for medical expenses, while Medicaid is a joint federal and state program that provides health coverage for low-income individuals and families.

One of the key features of SSDI is that it is an "entitlement" program, which means that individuals who are eligible for benefits are entitled to receive them regardless of the state of the program's funding. This is in contrast to other government programs, such as unemployment insurance, which are subject to annual appropriations by Congress and may not always be available to those in need.

Another important aspect of SSDI is that it is a "needs-based" program, which means that benefits are only available to those who are unable to work due to a disability and have limited financial resources. This means that individuals who are able to work or who have substantial income or assets may not be eligible for SSDI.

Applying for SSDI can be a complex process, and many individuals find it helpful to seek the assistance of a trained advocate or attorney. The SSA has a detailed application process which can be quite daunting to complete however is beneficial once approved. 

In conclusion, Social Security Disability Insurance (SSDI) is a crucial program that provides financial support to individuals who are unable to work due to a disability. SSDI is funded through payroll taxes and is available to eligible individuals who have paid into the system and can demonstrate that their disability prevents them from engaging in substantial gainful activity. While the application process can be complex and time-consuming, the benefits of SSDI can make a significant difference in the lives of those who are unable to work. It is important for individuals to understand their eligibility for SSDI and to seek help if needed in navigating the application process.