This button allows when modifying the schedule to see the effect of this change on the project's end date or a specific task date without having to go look at that task.
It allows you to choose which task you want to follow if you want to follow an intermediate task.
When you click the following window appears:
You can select the end date of the project or follow the date of a particular task with the drop-down menu.
This is a toggle button, when you click it, it activates end date tracking and when you click again it stops following the end date.
You will be able to see if the Track End date is activated in the Status Bar, at the bottom of the Software window:
