From Joomla! Documentation
On this tab there is a list of menu items. You can choose between On All Pages, No Pages, Only on the pages selected and On all pages except those selected from the List.
If the latter two options are selected a list will show with all the menu items on as shown in the screenshot above. This allows you to assign modules to some but not all pages, and by selecting the menu links that you want the module associated with you can customize on what pages modules appear/don't appear. See How do you assign a module to specific pages? for more information.