Hello Grumpy ol' Geezer. It just means you can access your mail from your normal e-mail application such as Outlook Express or Outlook. In other words, you don't have to log onto chess.com to access your chess.com e-mail.
If you don't use Outlook Express or any similar application, it's not relevant. If you do, then go to Tools -> Account Settings in whichever application you use and add your chess.com e-mail address as a new account. You'll need to enter the correct incoming (POP) mail server and outgoing (SMTP) mail server, which were probably sent to you when you upgraded your membership.
Is that a good thing? Will someone please inform a Grumpy Ol' Geezer what it is and how I can use it?