ROM stand for read only memory which holds instructions for starting up the computer.
The computer loads data from read-only memory (ROM) and performs a power on self test to make sure all the major components are functioning properly.
The computer loads the basic input/output system(BIOS) from ROM. The BIOS provides the most basic information about storage devices, boot sequence, security, plug and play capabilities and a few other items.
Unlike the main memory(RAM), ROM retains contents even when the computer is turned off. ROM is referred to as being non volatile.