Master Pages serve a purpose similar to a "pre-printed" page. For instance, if you want every page to have a flowery border and a footer with page numbers, you would put those items on a master page. Any document pages that have that Master Page applied to them will have those objects on them. They're uneditable from the document page... UNLESS you override them.
You override ONE item by SHIFT-CTRL clicking (SHIFT-COMMAND on Mac)that item while you're on the page. For instance, if you don't want the flowery border on page 9, go to page 9 and SHIFT-CTRL-click the border and delete it.
Or you can override ALL Master Page items on a specific page by clicking the flyout menu on the Pages panel and selecting "Override ALL Master Page Items." This will allow you to select/edit/delete any Master Page item from THAT page. Just remember, however, that your Master Page will still be applied to any new pages you create.
If you don't want ANY Master Page applied to ANY of your pages, go to the Pages panel, pull down the menu, click "Apply Master to Pages...", select "None" from the dropdown list, and enter "1-" in the "Pages" box.