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.
Joomla! Users and Administrators need to only understand the following:
It's possible to visualize all modules set on a template by calling the
"tp=1" query string on the front-end.
Joomla 2.5 and Joomla 3.x will show the template positions with ?tp=1 or &tp=1 if the Preview Module Positions mode in the Template Manager has been Enabled:
The image below shows an example of a module map when module mapping is enabled in template administration.