Entity iframe provides a simple way of creating and managing the embed-ability of content you are creating in a drupal site. It provides a path that displays an item in a different theme (optionally) which is more appropriate for visualizing in an iframe. There are several simple settings for modifying and controlling the output when viewing an entity as an iframe. This provides a consistent path for the rendering an entity in a way that's easy to be displayed in an iframe. An example of when you'd want to use this is when you have entities in a system that are objects for use in other systems (think youtube).