An active message is an Automated Workflow step that has been marked as ready to send.
When saved as active, anyone added to the group will get the message as determined by the scheduled timeline based on Group join date/time.
If a message is not active, no one in the Group can receive the message until it is made active.
This allows you to have drafts of a message before it goes live.
To learn how to make a message active or inactive and change the send status click here.
You can reactivate and deactivate as much as you like, but keep in mind that the ones marked in red will not go out, even when a person's join date matches the scheduled time. Only the ones that are active (marked by the green dot), that match the criteria, will be sent.