It's tree view panel.

A tree view or an outline view is a graphical user interface element (widget) that presents a hierarchical view of information. Each item (often called a branch or a node) can have a number of subitems. This is often visualized by indentation in a list.

An item can be expanded to reveal subitems, if any exist, and collapsed to hide sub items.

Tree views are often seen in file manager applications, where they allow the user to navigate the file system directories. They are also used to present hierarchical data, such as an XML document. Extended tree view is the central component of an outliner application.