Starting in watchOS 6, the Apple Watch has become a trusted device for Apple ID authentication purposes.
When you or someone else signs in to your Apple ID on a new device or browser, the Apple Watch will automatically alert you, complete with an approximate location of the person. If the sign-in attempt is allowed, a six-digit verification code will then appear to be entered on the new device or browser.
Something I haven’t seen before watchOS 6: the Apple Watch can now receive and display Apple ID Verification Codes as a trusted device for 2-factor authentication. pic.twitter.com/Oin8AbYEDc — Jeremy Horwitz (@horwitz) June 10, 2019
This functionality has been available on iPhones and iPads since iOS 9, and on Macs since OS X El Capitan, for Apple ID accounts with two-factor authentication enabled. Now, users simply have one more option in the Apple Watch.