1. Click on the cogged wheel icon in the upper right corner. (pic. 1)
2. Select the Semantic zoom function from Settings. (pic. 2)
When the semantic zoom is turned on it hides the less important concepts on the map. The hidden concepts and relationships are marked by blurred grey color. (pic. 3)
When you zoom out by pressing - or using a mouse wheel or touchpad gesture the marked concepts will disappear and only the important ones will remain. By zooming in the hidden concepts will reappear again.