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Apple ID is a universal identity system from Apple that they use on iOS and macOS systems as well as their iCloud service.
Fearmongering Security UI
Apple ID, in particular on MacOS 10.13 High Sierra, presents a very scary dialog if you don't go along with their TFA prompting (which just happens to force you to use SMS for TFA).
You literally have to choose the scarier option "Don't Upgrade" if you want to avoid / postpone / never setup SMS as your second factor.
See related: SMS#Apple_ID