Preview Panel
The Preview Panel represents the final view of what will be rendered when the scene is Played.
You can't directly interact with the Preview panel for editing purposes. If the scene has interactive nodes in it, then while the scene is playing, the preview panel will direct interactions within the panel (key presses, mouse interactions) to those nodes.
VR Preview
If the scene has XR Support enabled, then the preview panel will have a button to turn on/off VR preview. See the XR guide page for more information on XR and VR preview.