Before assessing the severity of a disability applicant's medical impairment, the SSA will look at whether the applicant is working and, if so, whether the applicant is earning too much income to be considered disabled. In addition, for SSDI (the disability program for workers who have paid Social Security taxes), the applicant has to be eligible for payments from the Social Security trust fund. For SSI (the low-income, needs-based disability benefit), the SSA will also look at the applicant's family income and assets to see if they are over the limits. And for non-U.S. citizens, the SSA will look to see if the applicant is in a qualifying alien category.
A person who is earning more than a certain small monthly amount won't be considered disabled, regardless of their impairment.
SSDI stands for Social Security disability insurance, and to be eligible for it, you have to have paid FICA taxes into the Social Security system for many years.
SSI stands for Supplemental Security Income, which a needs-based disability program for low-income people who haven't worked long enough to qualify for SSI.