ROM - Read Only Memory
Read Only Memory (ROM) is a form of storage. It is non-volatile - so it does not forget when the power is turned off.
It is written to (saved to) by the manufacturer and then can only be read - it can not be saved to.
In a computer they are used to permanently store its boot data - the programming that is needed to start the computer. This programming includes loading the operating system into RAM, controlling the major input and output tasks and letting users run hardware diagnostic (checks to see why it doesn't work).
This boot up firmware programming is called BIOS (Boot Input Output System).
ROM chips are not only used in computers but in most other electronic devices too. The video talks about how it is used in Smartphones, and compares it with RAM (in phones) too.
RAM vs ROM in Smartphones