Activation History shows a table with the entire activation history for a client.
The icon to the left of the name shows the status of the most recent send for that Activation.
Activation can be deployed multiple times. (i.e. the audience probably gets re-deployed after every update) Simply, a send is an instance of an activation. By clicking on the activation name, you can view send history per activation.
Destination shows where the audience was deployed to:
Google CMU (Customer Match Upload)
The CRM List column distinguishes where the deployed audience came from. This is most useful when deploying segments.
Size vs. Members Sent: These should match. Size represents the number of rows we have stored for an audience. Members Sent represents the number of rows that were successfully sent to the deployment source.
The Last Send column shows the date and time of the most recent send.