Option bytes
A small region of flash that controls how the microcontroller itself behaves after reset.
On STM32, the option bytes live in their own flash block outside program memory. The hardware reads them on every reset, before the first instruction runs, and they decide among other things the read-out protection level, which sectors are write-protected, the brown-out reset threshold, whether the independent watchdog starts by itself, and where the chip boots from.
Changing them takes two separate unlock keys, and the change only takes effect after a reload. The ceremony is deliberate: one slip can lock the debug port permanently or make the chip boot from the wrong memory. The AVR world calls the same thing fuse bits, and its forums are full of people who accidentally selected an external clock that doesn't exist.