From Joomla! Documentation
A module position is a placeholder in a template. Placeholders identify one or several positions within the template and tell the Joomla! application where to place output from modules assigned to a particular position. The template designer has complete control over module positions, creating variations between templates and the respective Joomla! default positions assigned to modules in the installation sample data.
For example, the module position Left could be defined to be on the left side of the template to display a site navigation menu. So if a module is assigned the Left position, it will be displayed wherever the designer puts that Left module position - not necessarily the left side of the page.
Understanding Module Positions
Joomla! Users and Administrators need to only understand the following:
- Modules are set positions dictated by the template developer when they developed the template.
- Modules can be turned on and off based on menu selection.
- Modules can be reordered in their assigned position using the Module Manager.
It's possible to visualize all modules set on a template by calling the
"tp=1" query string on the front-end.
- Extensions → Template Manager then click the Options button in the toolbar.
- Under the Templates Tab, make sure Preview Module Positions is set to Enabled.
The image below shows an example of a module map when module mapping is enabled in template administration.