A Sidebar is a column to the right of your survey with additional content, meetings or events, timelines & project milestones, documents, & external videos.
A sidebar holds for useful for information that you want users to easily reference at a glance (ex: people to contact about a project, dates for upcoming events, project timelines, etc.). This information can always be included in your project description, but some have found the sidebar to be a more visually appealing option for this type of additional info.
You can also add content, including videos and documents to a sidebar. You can add pretty much any content you like, with the exception of questions.