Sidebars always appear in the RightRail (the right-most column) of Content 3 Column pages. Sidebars are useful for providing ancillary information, helpful links, important call to actions, contacts, and more. Sitecore has four sidebars to choose from:

• Call to Action
  The entire sidebar area is clickable and the background is yellow.
• Learn More
  Intended to highlight content and send a user to another page on your site.
• Informational
  Displays general information that's purpose isn't a Call to Action or Learn More Sidebar.
• Related Content
  Uses Sitecore's content tagging to display lists of items with a pre-defined tag.