This button hides the tree view panel in the main LEX window if it's currently visible, and shows it if it's invisible:
To hide and show the treeview panel, you could also use the menu option View - Directory Tree.
Another easy way to hide (and show again) the treeview panel is a special button on the left outer edge of the main window. It appears only when you move the mouse near the left window border and don't move it for half a second:
When the tree view panel is invisible, you can move into a directory container by double-clicking on the regarding container object, you can change to the parent container of the current display container by pressing BACKSPACE or the Go up one level button .
However, when you use the deep tree object list view and your tree view panel is hidden, you cannot see easily in which container you currently are - in the address bar on the top of the window, there is always shown the currently marked object, not the currently selected container! If you want to find out what your currently selected container is, just show the treeview panel again.