A menu consists of various details, but for now I quote the principal. A menu consists of an action and a parent. The action corresponds to what happens when you click on the menu. Parent indicates the menu you're doing is a submenu of another.

When you put no parent, then the parent is always the root, that is, the menu that you create is in the top bar. When you create a menu whose parent is a menu that is on the top bar, then your new menu appears on the left side bar.

This ideology offers you tremendous flexibility, because, imagine the following: In the top bar you have two menus: "Messages" and "Contacts". After, create two submenus for "Messages", namely, "Messages Sent" and "Incoming Messages". This time you have the "Messages" menu with no father, that is father is the root, then it sits on the top bar. The "Sent Messages" and "Incoming Messages" have as parent the "Messages", therefore, appear in the left pane. If tomorrow you want to move all messages into the contacts, just you place the "Messages" with the father being the "Contacts" and all content will be moved into the "Contacts". Ie, can move all content just changing the parent of the top of the hierarchy.

Imagine the menus to be one family tree, to move the upper branch, all descendants will also be moved.I would point out one thing, if you do not associate an action, so the menu does not appear on the page.