Built into every computer is one or more chips that store program code which provides the basic input output fucntions for the system (aka BIOS). designed to power on the system, perform basic hardware tests (POST) and bootstrap the system. This program is stored in a chip which retains it's values even when deprived of electricity. The first ROMs were hard-wired. Their contents were 'burned in' at the factory and could not be changed. Frequently a bug was contained in the code that created problems. This actually happened with the early IBM machines. Their bootstrap ROM would identify the serial communication ports, but would erroneously assign the wrong memory registers and interrupts to the wrong COM port leading to COM port conflicts. The fix for this required upgrading the entire computer, or simply living with it.
Later, as technology advanced, flashable ROMs were created to store the bootstrap software. The advantage of the flashable ROM was that it could still hold it's value when deprived of electricity, but could also be updated. Instead of having to remove the chip and replace it, or having to scrap the computer and buy a new one, the ROM could be 'flashed' with new, fixed code to correct the previous bug.