Fixed income is a general term for someone that can’t change their income because it is provided by a social safely net. In my case, that’s the US disability system, SSDI in specific. You get a monthly income and that’s that. There is some wiggle room for other income, but if most of the people on SSDI could do enough work for that, they wouldn’t qualify for SSDI in the first place.
But it also refers to retired people on social security, and sometimes even people that get income from a pension.
In other words, the amount you get is not only “fixed”, there isn’t a way to increase it reliably.
There in the US, even the maximum SSDI amount you can get is below the poverty line. We’re lucky in that there are three adults on various SS programs, so we do have a little disposable income at the end of the month, but we’re talking maybe twenty or thirty bucks.