Server Name Indication is an extension to the TLS computer networking protocol by which a client indicates which hostname it is attempting to connect to at the start of the handshaking process.
The most import reason to have this extension is to allow a server to present multiple SSL web site, or multiple certificates on the same IP address and TCP port number. So one IP address can serve more than one HTTPS web sites.
This extension insert the host name in the very first request sending from client Hello header. The standard TLS will send host name after handshaking.