Your security key unlocks your encrypted chats on a new device. Wiply keeps it safe for you, so on wiply.net you never need it: every browser you sign in to unlocks your chats automatically.
You need it only to use the Narendi app at app.narendi.com for the first time. To see it:
Never share your security key. Anyone who has it can read your chats. Wiply staff will never ask you for it.