The Scene tree is a tree view of all nodes in the scene, arranged by their parent/child relationships. The arrow on the left of a node allows you to expand and collapse the list of children for that node.
Each node type comes with its own icon.
Regular node (Transform node)
Drag and drop to reparent nodes (changing the parent/child relationship). Nodes that have the lock sign cannot be reparented.
Ctrlfor multi-select on the scene tree
Hover over a node and click the
icon to toggle its visibility. Hidden items are not visible or selectable in the scene, but can still be edited in the Timeline and Graph Editor.
By default, toggling the visibility of an item also toggles the visibility of all its children.