SSI means "Supplemental Security Income." It is not a form of Social Security. The reason why it is administered by the Social Security Administration is because they are set up to evaluate for inability to work due to disability, or advanced age. SSI is a form of federal income support to the poor who cannot sustain "substantial gainful employment" because they are too disabled or too old. Each state can add to the SSI check if they want, just like some states add to the amount of food stamps that people get.