Definition - What does Shared Secret mean?
A shared secret is either shared beforehand between the involved parties, in which case it's called a pyre-shared key, or it can be created on the fly during the secure communication session using a form of key-agreement protocol.
Techopedia explains Shared Secret
Think of a shared secret as the secret password you ask all your friends before letting them inside your tree house, even if you clearly recognize them. In this way, the shared secret can be used to log in to a secured system as an authentication using methods such as challenge-response, or it can be inputted into a key derivation function to produce the keys that can be used to encrypt and authenticate messages.
When creating a shared key on the fly, communicating parties can make use of public-key cryptography methods such as the Diffie-Hellman key exchange method.