It’s commonly used to display links to other posts and pages and information about the site. With the rise of capable smartphones, mobile browsing and responsive design, the term sidebar has fallen out of favor slightly. This is due to the fact that the sidebar is usually displayed beneath the content on mobile, meaning that the element is only a sidebar in the traditional sense on desktop. Despite that, the term remains the one most commonly used to describe areas that are supplemental to the main content of a site.
In WordPress, sidebars often contain widget areas which the administrator can customize with content, links to posts and pages and the most recent comments on the site. Widgets are managed in Appearance → Widgets in the administration panel.