In web design, a sidebar is a vertical column that appears alongside the main content of a webpage, typically on the left or right side. It is used to display additional information or navigation elements such as links, advertisements, contact information, recent posts, or search bars. Sidebars help organize content and provide easy access to other parts of the website without cluttering the main content area. They play a significant role in improving user navigation and enhancing the overall layout and functionality of a website.