It negotiates an SSL connection, step by step, and prints debugging info in excruciating detail. It also includes a dump of the server-side SSL certificate in PEM format. You can use that certificate to test offline, import into your client-side keystore, or anything else.
For example, say you’re troubleshooting a connection to https://example.com/. You can use this (*nix) command line to connect on port 443 and examine the SSL handshake and server-side certificate:
openssl s_client -connect example.com:443