TOPIC: Menu or menu item?

I would like to add to the main menu a heading ie " Different Inspections"  and have that clickable to expand and list all the different types of inspections. rather than have one long list plus all the other pages on the site It makes the sidebar too long .
Does anyone know if this is done by adding a menu or is it done by adding a menu item?  the Pages are currently parented under navigation so as I understand it I will then go back and reassign them to the the new menu.. Menu item?

I have largely finished adding my content now I need to organize it.  BTW many thanks to the posters on this site. With some help from Dominic directly and many readings of these posts I have so far got quite a bit done.

Add a menu item under a menu. Then you'll also edit that menu item and click the Expanded box if you want it always expanded, or don't if you want them to click it and then it expands.

Then just make sure that your pages have the above menu item as their parent in the menu section when editing the page.