When designing a web block you use the Placeholder widget to define areas of the web block that are to be designed later on when the web block is used. This allows you to have web blocks with content that is only designed on the context where the web block is used.
Placeholders are specially useful to design web blocks for screen layouts (also known as page templates or master pages) used in Themes to create and scaffold screens and other functionality. Learn more About Themes, Create Web Screens, and About Scaffolding a CRUD.
About Web Blocks | Use a Web Block