This panel displays the current scene tree. You can choose to hide or show meshes (geometries) using the bottom menu. The scene tree also displays materials assigned to meshes:
The nodes are displayed gray, while the meshes are white. Materials are at the right of the meshes and can be hidden using the View menu at the bottom. You can hide and show nodes by checking the boxes next to them.
You can also display a popup menu by right-clicking the node in the scene tree:
Here’s what you can do with the menu:
You can also access some of these items using the Edit menu in the main menu bar.