what does flash system ROM does

Can you rephrase the question, it doesn't make any sense ?
If you are asking about BIOS and flash memory (CMOS) than it's like this.
BIOS is a ROM (Read Only Memory), actually EEPROM (Electrically Erasable Read Only Memory) chip which could be reprogrammed by what's called "Flashing" it.
There's also some flash memory in it which keeps all settings for BIOS and is kept "alive" by on-board battery.