The sidebar.php file is used to display the primary and secondary widget areas. WordPress widgets are like a plugin, but designed to provide a simple way to arrange the various elements of your sidebar content (known as “widgets”) without having to change any code.
Following is a description of the contents of the sidebar.php file in the theme:
The sidebar.php file defaults to a set of widget displays, It uses a dynamic_sidebar function which will display the default list of widget displays if not active, otherwise the widget information you set up in the dashboard page will be displayed. Various options are available. For the Starkers theme, a secondary widget area is set up and available for use. By default, this has no content.
Sidebar widgets are used to display such information as a search bar, a list of site pages, a list of recent comments, a list of archives files, and so on. There are many pre-defined widgets available for download.