WordPress themes divide the areas of the screen into several different sections. Those sections that can be used with widgets are different than the section used to display content, and a sidebar is one of those sections.
Normally, a sidebar is a column that appears either to the left or the right of the main content, but its exact location depends on the theme that you are using. Technically, a sidebar can appear below the main content or above it and be either in the form of a row or a column, but the most common usage is a column to one side of the main content.
Some themes come with multiple sidebars, which can be given logical names like “left sidebar” and “right sidebar” or simply numbered like “sidebar 1,” et cetera. With some themes, you’ll just have to add widgets to each sidebar in order to determine where they will show up on your website.
It’s also important to understand that sidebars can display differently depending on whether a visitor is using a desktop computer or a mobile device. Always test each size layout after making any changes to the content in a sidebar.
One common use for sidebars is to display advertisements because these will appear adjacent to the main content on your site. If you’re interested in monetizing your WordPress site, you’ll need to contact the advertising company in order to get the proper codes for displaying an ad in your sidebar.
In order to modify the content in your sidebar(s), log into the administration panel and then click “Appearance” on the left-hand column and then “Widgets.” From there, you can add, delete, or modify the widgets that appear in your sidebar(s).