When you create a new page, by default it will be a top level page such that it is at the top of your page hierarchy. To make it easier to find pages when you have more than 20 though it helps to arrange pages into parent and child pages or if you prefer top level and sub pages.
Changing the parent of a page does not affect where it appears in the drop down menus. If you want to change the order or positioning of a page within the drop down menus then change it in Appearence > Menus.
Top level pages have (no parent)
Select a parent page from the list
Select the parent and update the page
The Order lets you sequence a set of pages in the WordPress dashboard in some order other than alphabetical. It won’t change how the web pages are ordered or arranged to the people viewing your website but it can be useful if you have a lot of parent and sub pages and want to organise them logically.
The larger the Order number the further down the list a page will appear. Order numbers DO NOT have to be sequential. For example you could have 1, 10, 25, 40, 102 as the order numbers for 5 different pages and the sequence would follow that order just fine.