If you have sent any Activity Alerts, you will see details about each one here.
The history is shown with the most recent Alert first.
The Delivered row shows how many people received the Alert. The total is shown first, followed by how many people received the message Online (they received an IM), then how many people received an email message, and how many people received an SMS message. An SMS message is a text message sent to a cell phone or pager.
The total number of people subscribed to the list (at the time the Alert was sent) is also shown. The total subscribers will usually be more that the total delivered, since some people will not be online, and they didn't want to get offline (email or SMS) notification.