What does 'unable to verify the first certificate' mean in an OpenSSL s_client connection?

The error 'unable to verify the first certificate' in an OpenSSL `s_client` connection indicates a problem with the chain of trust for the server's certificate. It means the client cannot successfully validate the server's certificate because it's missing a trusted intermediate certificate or the certificate chain is broken. The client's trust store (typically the system's CA certificates) doesn't contain the necessary certificates to verify the server's identity.