A WordPress sidebar is an area to show widgets from dashboard section Appearance > Widgets.
Sidebar content is not considered a part of main blog content and usually remains static. Sidebars are not always vertical and could be horizontal also although WordPress vertical sidebar is more popular. See a layout below.
You can place valuable blog information in sidebars in the form of widgets like recent articles list, social Like buttons, product promotions, polls and related stuff.
See learningcms WordPress sidebar example below