As the name suggests, it is designed to let you keep an eye on all the activity that goes on your own Google account.
If you sign up for this service, Google would offer you a new updated report every month that would highlight activities conducted on various Google services during the last 30 days.
You get a password protected file with stats and other information related to your usage. Google added:
Knowing more about your own account activity also can help you take steps to protect your Google Account. For example, if you notice sign-ins from countries where you haven’t been or devices you’ve never owned, you can change your password immediately and sign up for the extra level of security provided by 2-step verification.