Placeholder content in a text frame or picture frame that is inherited from a master page can be edited on your publication page without affecting the master page itself.
As opposed to editing the master page itself, which affects all publication pages to which the master page is applied, you can edit the contents of any frame on the publication page, making it unique for that page. This 'local' edit will not be made on the master page itself. Placeholder content could be empty text or picture frame, or frames filled with filler text or a temporary image, respectively.