Something fixed is rigidly set or placed. A fixed point does not move, as in this quote: “She was his North Star, the fixed point round which his world turned” (from David Gemmell’s Fall of Kings).
Anything described as fixed is unchangeable. A fixed rent can’t be raised by the landlord because it’s legally set at a certain amount. That’s handy if you have a fixed income –- one that stays the same every month no matter what. A fixed piece of wood is nailed to the wall. Fixed things are stable and constant. A fixed stare doesn’t look away. Fixed also means “mended” like if you fixed a broken chair before anyone sat.