SSDI or SSD stands for Social Security Disability Insurance. It is a government benefit administered by the Social Security Administration (SSA). SSD is a monthly benefit that provides relief for people unable to work full-time due to mental and/or physical issues.
One way to think about the benefit is going on early retirement -- but, of course, not by your own choice.
You are eligible if it is determined you are Disabled, or not "competitively" employable.
Your monthly benefit amount is tied to how much you paid into the system over the years. You can easily find out what your monthly benefit will be by using your online my Social Security account.
Essentially, the SSDI / SSD program is a government run insurance policy. When you work, your employer withholds and pays the premiums on your behalf, as part of the Federal Insurance Contributions Act (FICA). When it works, it is a beautiful system -- when it doesn't, you need a competent attorney to appeal.