So what can you do with it/offline Gmail features? Well, if the tool is already activated in your account, you won't be worrying about having no Wifi or Internet connection. With offline email access and Gmail's offline web application API, you will be able to read, star, label, archive and compose new mails. On the downside, as of yet, you won't be able to execute file attachments in the mails.
How to activate offline Gmail? Although it is still in the experimental stage, you can do so by going to your Gmail labs. You got to click on the "Settings" and then the "Labs" tab. After that, select the Enable button next to the Offline Gmail feature. Save the changes and you're done. For more details on this, you may head on to blog.wired.com and go through the "..Gmail Releases New Offline E-mail Tools" article.