Firmware
Firmware is something in-between "hardware" and "software" - hence the name "firmware".
Hardware, in high-tech devices like phones, is any part of the phone that is physical and cannot be changed unless physically replaced. For example, a 4G-only phone cannot be upgraded to support 5G. Only a phone designed for 5G, with 5G chips and such inside, can access a 5G network.
Software, meanwhile, refers to things that can be changed, and usually easily, like adding a new app or updating your OS.
Firmware is software that sits at a very low level, below the OS. It may provide the interface layer between specific hardware and the OS. It is sometimes more difficult to update than software and is typically not updated as often, if ever.
One type of firmware (boot firmware) is responsible for loading the OS upon startup. While the OS controls nearly everything while it is running, something must come before that when the device is first turned on, to get the OS started. That's firmware.
Firmware can also be used to disable certain device features in a semi-permanent way. If a manufacturer wants to offer several variations of a device with slightly different capabilities, it may be cheaper to manufacture and certify only one hardware design, then tweak the firmware at the factory to create several different variants of the device. This is done frequently with phones to be sold by certain carriers, to limit the phone to only work with the specific network frequency bands used by that carrier.
