An activity trigger is set up against an activity due date.
There could be numerous reasons for an activity becoming overdue, the assigned resource may be overloaded with work or may be sick. Timely escalation helps in taking actions in time.
An activity trigger is executed when the time period relative to the activity due date has past and as soon as the activity becomes pending or is taken. However, you can set the activity trigger to execute before the activity due date is reached. A trigger cannot execute for an activity that is either complete or on hold. For example, you may need an alert 2 days before an activity due date if no resource is picking up and completing the activity (which means the business process is not progressing as it should).