what does topLevelItem in QTreeWidget means?

[opengpu2]

what does topLevelItem in QTreeWidget means?
i read the doc, but still don't understand what it really mean...

[andreyc]

Top level items in a QTreeWidget do not have parent items.
Take a look on the pictures in QTreeWidget description
The most left items on each image are the top level items.
Not sure if forum will show my drawing correctly

invisible_root
    top_level_1
            not_top_level_1
            not_top_level_2
    top_level_2
            not_top_level_1
            not_top_level_2

Invisible_root is not visible in QTreeWidget and is accessible by QTreeWidget::invisibleRootItem()

[opengpu2]

so u mean when i write like this: pRootItem = new QTreeWidgetItem(this);
pRootItem is just top_level_1, and not invisible_root...
why do Qt add an invisible_root here?
and what things will the top_level related API make easier?