Content Holders can be used to re-use and populate the same page elements, or content snippets on more than one page or page template. Content holders can save you time and energy, and also allow you to update many pages in one place. This can be great for injecting scripts and other infrequently changed content.
Keep in mind that when you edit the contents of the content holder, the changes are reflected on all the pages that share the content holder.
For example, if you wanted google code injected on every page in the HEAD (the Head is not displayed content in the browser, its used for loading scripts and setting variables for the page), then Content Holders make it a breeze.
This brief video will show you the steps create a content holder