The slack date displays the date at which each task will impact the completion date of the project.
Predecessor relationships and task constraints will increase the slack time. (This is the difference between the slack date and the projected completion date of the task.)
For critical path tasks, the slack date will match the projected completion date unless the task is already late or behind.
For non-critical path tasks, the slack time will increase the earlier the task is scheduled and the slack date will remain in the future until the task becomes so late that it begins to impact the completion date of the project.