Hierarchy

The object tree of the project. An object inside another follows its parent in position, rotation and scale.

Select: Click the item. Ctrl adds to or removes from the selection, and Shift selects the range.
Reorder and change the parent: Drag the item into another one or between them.
Group and ungroup: From the context menu.
Hide: With the eye icon.
Lock: With the lock icon. A locked object is not selected or moved in the viewport, and locking a group locks what is inside it.
Rename: Long click on the name.
Search: Filters the list by name.

With several objects selected, the context menu applies to all of them.

A module object shows as unavailable when its module is not loaded, and can only be renamed or deleted.

An asset shows as a single item, with its own icon.

Resources

The materials and textures of the project. The same material can be on several objects, and changing it changes all of them.