The panel on the left side of the RViz is called Displays panel. The Displays panel contains a list of display plugins of RViz and its properties. The main use of display plugins is to visualize different types of ROS messages, mainly sensor data in the RViz 3D viewport. There are lots of display plugins already present in RViz for viewing images from camera, for viewing 3D point cloud, LaserScan, robot model, Tf, and so on. Plugins can be added by pressing the Add button on the left panel. We can also write our own display plugin and add it there. The detail of tutorials for writing a display plugin on RViz is available at http://docs.ros.org/jade/api/rviz_plugin_tutorials/html/display_plugin_tutorial.html.