If a ticket is opened and has time tracked on it but nothing is submitted, then that ticket is considered as unattended. If that ticket wasn´t assigned to an agent by the time the agent opened it, then the agent will receive an Unassigned Unattended Ticket point for opening and close the ticket without actually doing any work on that ticket.
Agent opens ticket #123 which is assigned to Zendesk Group Support but not to an agent
Agent closes ticket #123 (1 Unassigned Unattended Tickets point rewarded)
The Unattended Metric is useful to help agents that may be cherry picking unassigned tickets. I.e., browsing through views, opening a ticket and looking at it but taking no actual action such as submitting a comment, assigning the ticket, or changing the status.