What is the JWT secret key?

Excuse me, could you please elaborate on what the JWT secret key actually is? I'm somewhat familiar with the concept of JSON Web Tokens, but I'm still unclear about the role and importance of the secret key in the JWT authentication process. Is it a unique identifier that's used to validate the authenticity of the token? Or is it something else entirely? I'd really appreciate it if you could provide a concise yet informative explanation of the JWT secret key and its function in the overall security of the system.