What is sharedSecret in JWT

In Silhouette, a JWT authenticator could be created using the following class

JWTAuthenticatorSettings(fieldName: String = "X-Auth-Token", requestParts: Option[Seq[api.util.RequestPart.Value]] = Some(Seq(RequestPart.Headers)), issuerClaim: String = "play-silhouette", authenticatorIdleTimeout: Option[FiniteDuration] = None, authenticatorExpiry: FiniteDuration = 12 hours, sharedSecret: String)
What is the use of sharedSecret? Is it what the application would use when signing the JWT header/claims payload?

From the documentation: https://www.silhouette.rocks/docs/config-authenticators#section-jwtauthenticator

The shared secret to sign the JWT