Joomla works on Positions. It basically divides the page into different parts that is known as positions. Module are assigned to those positions. To display the module positions you required to follow the following points:
Login to the backend of the Joomla site.
Go to Extension -> Template Manger. Select Options on the top of the page.
Enable the Preview Module Positions
Now go to URL field of your browser and enter domain name of your browser and append "?tp=1". For example, http://domainname.com/index.php?tp=1.
You are able to see the module positions outlined red.