It's "supplemental" income, not enough in itself to make you rich enough to pay taxes Social Security benefits, however, can.

"While most recipients will not get as much, the monthly maximum federal SSI payment for 2013 is: $710 for an eligible individual"
From: http://ssa-custhelp.ssa.gov/app/answers/subtopic/c/48

It's on the "bottom," not really income but like food or your housing, just there/a necessity.