The GMail team announced that the https connection will be used by default when you login into your Gmail account. This option is not new and is already exist since 2008. So, what is HTTPS? An excerpt from Gmail’s help:
HTTPS, or Hypertext Transfer Protocol Secure, is a secure protocol that provides authenticated and encrypted communication.
It is good that Gmail enable it by default from now on. But there is a drawback, using https connection is slower that using the normal http connection. But for better privacy/security protection, I guess that it is better to use https connection.