The timeline activity on a work card refers to the time since the last email was added to the work timeline.
Actions that update the Timeline Activity
Adding a note
Updating an existing note
Sending an email from the work item
Sending Client Task Emails
Actions that do not update the Timeline Activity
Renaming the work item
Adding a comment/(Assigning to a colleague or yourself) to the Client task Emails
Adding a comment/(Assigning to a colleague or yourself) to an existing note
Adding a Comment/(Assigning to a colleague or yourself) to the email sent in the work item
Pinning/Unpinning a note/email/Client Request Email