This icon indicates Denied Time and it automatically appears for any period where the Scheduled Time was more than zero, but where no Actual Time was recorded. If you create a Tracker entry with a Scheduled Start and Stop time but no Actual time, the Denied Time icon will be turned on.
When recording a Tracker entry with a Scheduled Start and Stop time but no Actual time, the system will ask you if you would like the Visit Type set to 'Denied'. If you want the visit to count as a 'Denied Time' event in the Statistics, mark the visit as 'Denied'. If you don't set the Visit Type to 'Denied', the Denied Time icon will still appear but the visit will not be counted in the Statistics as a 'Denied' event.