ROM is an old term for “Read-Only Memory”. Nowadays these chips are often more correctly referred to as “non-volatile memory”, but it boils down to the same thing: they’re chips that store data even after you unplug your computer. When a digital device turns on, it effectively has amnesia. The only information it has about the world is what’s stored on its ROMs. So the first thing many devices do when they wake up is start reading instructions from a ROM.