What are module positions?

From Joomla! Documentation

Site templates divide the "pages" displayed on a site into a series of module positions, each with a different name.

Extensions => Templates. Select the template you wish to preview and click the edit icon. Click the preview icon.

You can add or remove positions by modifying your template html.

You assign a module to a position using the module manager.

In the module manager, edit the module. In the left column select all, none or the specific pages you would like it to appear on.

Notes: Modules that are not enabled will not display. Modules can be assigned to unused positions (positions not in the template) if you want to have them published but not displayed in a position (for example, if you want to display a module in content using {loadposition}). Multiple modules may be assigned to the same position. They will be displayed in the order shown for modules in that position in the module manager. If you want to display a module in more than one position, use the module manager to create another copy. Use the new icon and create another instance of the module.