Date type
Specify whether the trigger will be an "exact date and time" or an "anniversary." Selecting "anniversary" will create a trigger that is repetitive on the annual anniversary of the date selected in the Date field. A good example for selecting "anniversary" would be if the field contains the date on which the member was born, and you would like to send them a message on their birthday (the anniversary of their date of birth).
Date field
Select the corresponding date field from the drop-down menu. These correspond with the date tables, for instance DateBounce_ or DateJoined_.
For more information on how triggered segments work, see Segments: New Triggered Segment.