Google Account Activity, opt-in feature for your Google Account that sends a link to a password-protected report with insights into your signed-in use of Google services on a monthly basis after Signup has been merged with the Google Dashboard. Search engine giant started notifying users who signed up for this via email.
See your Account Activity information on Google Dashboard
You can now see your Google Account Activity information on Google Dashboard (check the screenshot below).
Though till this May, your reports will be available for viewing on the account activity page, which is currently showing this notification "Account Activity information merged with the Google Dashboard to provide you with a single point for account activity information and to provide more up-to-date information. Your existing Account Activity reports will continue to be available here until 31/5/2014."
And monthly reports you receive will be linked to Google Dashboard. "The monthly emails that you received to notify you of new reports will now link to the Google Dashboard, which will continue to improve over time to provide a similar experience to the Account Activity reports", last paragraph of Email sent by Google says.