The method of authentication Apple uses to identify the registered owner of a device.
Apple IDs contain personal information and a user’s preferred settings. When you add your Apple ID to a device, it automatically uses the settings and information associated with your Apple ID. For example, let’s say you purchase a new iPhone. When you sign in to that new iPhone with your Apple ID, your new phone will immediately be set up exactly as your old iPhone, complete with photos, texts, and emails if you are using the cloud.
You should protect your Apple ID with a secure password to keep it safe from cybercriminals.