[cesium-dev] Popup box like in Sandcastle example for GeoJSON simplestyle

What you are refering to is the InfoBox widget, which ships with Cesium. It can be used independently but it’s built into the Viewer widget and that’s where most people end up using it. Data loaded via GeoJSON/CZML/KML into the Viewer will automatically use the InfoBox to display description information. Check out the Visualizing Spatial Data tutorial for more information