The activity report provides data on how recipients interacted with the message, aggregating all the data throughout the history of the message.

Here is the information available:

Subject, message ID, first mailing date, last mailing date, tags associated (if present) and how many times the message was sent (with a link to the sendouts list)
The total number of recipients, obtained by summing the number of recipients of each sendout.
The total number of delivered emails and the delivery rate
The aggregated number of bounces

Let's do a rundown of which metrics are available for a message.

Opens

% Openings under privacy: This percentage indicates who received the message on Apple Mail from mobile or desktop, out of the total deliveries. Click on "Calculate" to view it
Unique open rate for the message, calculated over delivered emails
Opens are the sum of all unique opens registered for each sendout. For example, a recipient who received the same message twice and opened both occurings, will be counted two times in the opens.
It is possible to view the data on openings under privacy, from Apple Mail. Click on "Calculate" to view the data.
The opens number also includes recipients who opened without downloading images - detailed below as without images.
Total opens are the sum of every open activity by the message recipients
Segment opens is a link to a page where you can get open data sorted by recipient filters

Clicks

Unique click through rate for the message, calculated over delivered emails
Clicks are the sum of all unique opens registered for each sendout. For example, a recipient who received the same message twice and clicked on both occurings, will be counted two times in the clicks.
Total clicks are the sum of every click activity by the message recipients
Segment clicks is a link to a page where you can get click data sorted by recipient filters

Unsubscription

Unsubscription rate for the message, calculated over delivered emails
Unsubscriptions is the number of recipients who unsubscribed directly from this message.
Abuse complaints are a specific unsubscription type, which happens when a recipient reports a spam abuse to the email provider. For more info, read Types of bounces.
Total shares are the sum of clicks on the special links "Social share" and "Send to a friend"

The last part of the Activity report contains:

Open and click trends, with a default period of 10 days from the last mailing. The period can be changed to the last 10/30/90 days or to a custom period.
Bounces graph, with a default period of 10 days from the last mailing. The period can be changed to the whole period from the first mailing or to the last 10/30/90 days or to a custom period. This section also includes the number of bounce occurences and their rate over the email delivered in the selected time period.

Filters and groups selected for the last mailing
Details on the sender used for the last mailing