Tax exempt income.
State and municipal bonds are tax free to encourage people to buy them. The state is able to borrow money at lower rates to build schools, roads and infrastructure, and you the earn tax free income, albeit at a lower rate than other investments.
BUT. SSA, when computing your income, adds your tax free income back into your AGI so they can charge you more. They call it Modified Adjusted Gross Income.
This is irony: At this holiday season the IRS gives you the gift of the MAGI.