Real Time (Opening The Record) triggers duplicate check on the fields enabled. Basically, every time you open the record - it will query the database to see if there are any duplicates. If there are duplicates - it will show a warning icon which can be clicked and the duplicates will be displayed. If there are no duplicates - check mark icon will show up confirming that there are no duplicates. This feature can be turned ON or OFF. Please see "Passive (Click the Icon)" section as it directly relates to this one.