"signin.samsung.com/key" feature is a remote authentication tool designed to simplify logging into a Samsung Account on external devices, particularly Samsung Smart TVs
It may also refer to a fake “key” or login code requested by scammers pretending to be Samsung support.
If you are still signed in on any Samsung device (TV, phone, tablet):
Cross-Device Handshake: The user visits the signin.samsung.com/key portal on a secondary device, logs into their Samsung account, and enters the displayed key to "handshake" the two devices.