Sidebars are widget areas that can be filled with widgets.
Widget areas

Customizer options
Disable sidebars
If this is enabled, no sidebars will be displayed, regardless of the page or it’s template. Overrides everything.

Show secondary sidebar
Whether or not to show the secondary sidebar on all pages (with the exception of front-page).

Note: This can be overridden for specific pages by selecting a custom template from the page attributes.

Primary sidebar position
Allows to change the position of the primary sidebar. If secondary sidebar is enabled, then it will simply go to the opposite side.

Page templates
Default template
Page contents will be contained to a reasonable maximum width, sidebars are chosen based on Customizer options.
Full width page
Page contents will fill the entire display size, no sidebars will be displayed. This is useful for pages with more complicated design elements, such as the front page. Text should be manually wrapped in a container shortcode.
Page with no sidebars
Page will not show any sidebars, even if any are enabled in the customizer.
Page with one sidebar
Page will show only the primary sidebar, even if secondary one is enabled in the customizer.
Page with two sidebars
Page will show the primary and secondary sidebar, even if the secondary one is not enabled in the customizer.