A block is a piece of content that can be inserted into a column instead of using the WYSIWYG text editor. Doing so allows the content to appear on many pages while only having to be updated from one place, making it easy to maintain consistency.
If you have multiple pages that will have the same content, a content block is a great solution.
Select a folder by hovering over the asset name and clicking on the arrow (>) that appears to the right of the asset name. The contents of that folder will display on the right hand side of the page.