Let's take a new starter email campaign for example:
The number of actions at the moment in time that have been triggered. For the above example this is 3 emails sent and 3 pathways shared on day 0. 3 Welcome emails sent on day 1 and 3 emails sent and 3 resources shared on day 10 = total 15 actions.
3 people in total have started the campaign.
There is no one current. If someone was added to the campaign tomorrow then they would be marked as current.
3 people have worked their way through the entire campaign and are now marked as 'completed' (got sent all the actions from day 0 to day 10, so the above example means 3 people have been sent the day 0, day 1 and day 10 actions) so won't get any more communication. They may not have necessarily opened or clicked or done any actions from the campaign but they are completed as they have been sent the messages/actions.
Once someone is completed they will not get any more messages and therefore will not get any updates as they have finished the campaign. Therefore if you were to add new messages they won't get it as they will be marked as completed.
Remember the summary is for the entire campaign. To see specific users and if they have opened the email or clicked the call to action button on your campaign, select the reports tab.