It's like a personal Google Analytics, but it's less detailed and it focuses on security features. For example, Google shows a list of locations and browsers detected when you've signed in. If you never use Opera, but the browser is included in Google's list, then it's likely that someone else found your password. The activity page also includes the number of emails you've sent and received, the number of Google searches and stats from Google Latitude, Picasa Web and YouTube.
I've enabled this feature and Google says that my "next report will be ready in a few days" and I'll receive an email notification when it's ready. It would be nice to aggregate the data in real time and integrate Account Activity with Google Dashboard, which already shows some stats.
{ via Google Blog }