Apple is pushing you to adopt two-factor authentication for your Apple ID when you upgrade the operating system on your iPhone or iPad. During the upgrade process, you see a screen like this:

  Screen image of Two Factor Authentication Setup in iOS

We are very much in favor of two-factor authentication, but you should understand what you are doing before you set it up. In fact, if you enable two-factor authentication without understanding it, you may lose access to your AppleID account and any devices associated with it. So if you are unsure about two-factor authentication, select the option at the bottom of the screen that reads Don't Use Two-Factor Authentication. Once you educate yourself about this valuable security service, you can turn it on on your own schedule. Here is more information from Apple:

https://support.apple.com/en-us/HT204915

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