Read-only memory, or ROM, is computer memory that stores information that can be read but not normally changed or erased. Think of it like a book that's been printed and bound: you can read it as many times as you want, but you can't rewrite the pages.

When you turn on a computer, smartphone, or video game console, ROM contains the essential instructions that tell the device how to start up and what to do first. These instructions need to stay exactly the same every time, which is why they're stored in read-only memory rather than regular memory that can be changed.

ROM is different from RAM (random access memory), which is like a whiteboard that gets erased when you turn off the power. ROM keeps its information even when the device is unplugged or turned off. That's why your computer still knows how to boot up after you've shut it down: those startup instructions are safely stored in ROM.

Video game cartridges for older systems like the Nintendo Entertainment System used ROM chips to store the entire game permanently. When you plugged in the cartridge, your console could read the game data, but you couldn't modify the game itself (which is why you needed passwords or battery-backed memory chips to save your progress in some games).