A sidebar is a section of your website that is generally displayed along the left or right side of your posts and pages. It can also appear in other widgitized places, such as the footer. You can also have multiple sidebars in your site based on the WordPress theme you use.
Most sidebars are widgetized especially with the latest WordPress themes. This means it would be very easy for you to manage the contents or whatever you want to appear on your sidebar without having to mess with programming codes.
Here are helpful tutorials for managing widgets on your sidebar: