This new feature makes use of Google Gears to download a local cache of your mail, so whenever you are lacking an Internet connection you can still browse and read your mail as well as saving drafts to be sent later, mimicking the functionality of a true desktop application. Unsurprisingly the new offline feature is being touted as "experimental", while at the same time mentioning they have been using it in-house for months now.
Gmail is easily the most dependable and overall best free webmail service currently available. You may recall Gmail was launched back in 2004 as a closed beta offering a staggering 1 gigabyte of free storage (up to 7GB+ now). Somehow the "beta" label still remains to this date, despite of the fact Gmail is a far better solution than other similar established services like Hotmail.
To activate the new feature, go to your Gmail account, click Settings and then go to the Labs tab. If the feature is not there yet, Google says they will be adding it to all US/UK accounts during the next few days.