Positions are the section of your page in which you place Modules for things such as Menus, Advertisements, Archive Article lists, Calendars, etc. The name and placement of the Positions is st by the Template that you are using.

To find out the names of the Positions and the location on the page you can do one of the two following steps.

Method A:

Step 1: Navigate to Template Manager under the Extensions dropdown. Along the top you will see the available Actions.  Click on the Options button and you will be taken to the Global Configuration Screen for the Template Manager. In the Templates tab, select Enabled for Preview Module Positions. Select Save & Close.

Step 2: You will be brought back to the Templates Manager screen.  In the upper left, select Templates. You screen will show a large Thumbnail image of the various Templates that are installed on your site. Find the Template that you are using and select Preview under the name of the Template. With some 3rd Party Templates, this preview is not available.

Step 3: You will then be taken to a new tab with a rather distorted view of your website.  Each Position that is available for that Template will be displayed in red text and placed in the location it will reside on your site.

Method B:

Step 1: Follow the same steps for enabling the Preview Module Positions as in Method A.

Step 2: Open your site in a browser window to any page.

Step 3: At the end of the URL in the browser window, add the following code ?tp=1

Example: http://www.mygreatsite.com?tp=1

Step 3: Your web page will now show a rather distorted view of your website.  Each Position that is available for the current Template for that page will be displayed in red text and placed in the location it will reside on your site.

Some Templates will use Position Names like position-1, position-2, other Templates will use more descriptive names like top-right. 

Below is the Position layout for the Protostar Template that comes as the default template for Joomla 3 installations.

Positions for the Protostar Template