When you have a number of activities, some of which are linked to others and some of which may need preparation time, or follow-up afterwards, or may have to be repeated, it can become difficult to keep track.
When you click on the Overview of current output tab, you will get an overview of all the activities of the current output in the form of a Gantt-chart.
The two drop-down lists at the top right allow you to manipulate the view of this Gantt chart.
On the left, you can see the list of key moments (shown first in blue) and activities. On the right, you can see the actual Gantt chart:
Alternatively, if you want to have an overview of the activities and/or key moments of all the outputs - or in other words the complete planning of your project - you can open the Planning window.