Where modules sit on a page is controlled by Module Positions.
Think of your website layout as a grid of empty boxes. Take a look below at the standard box arrangement for websites that we create.

Each box has a specific and unique name assigned to it. Common names include : title, left, Component, right, bottom1,2,3,4 and footer1 and 2.
If you tell Joomla to publish a module in the right position, it will appear in the corresponding box space on your website.
Focusing on the 'Right' Position
The right position (often referred to as a sidebar) is one of the most common and effective places to put modules.
Why use a sidebar?
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It’s excellent for secondary navigation (sub-menus).
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It keeps important, non-essential information visible without cluttering the main content area.
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It is visually balanced for many standard website designs.
- As the content might be non-critical, on mobile devices, it will sit underneath the main content.
So we now have a module, and we've told it to appear on the right of the page. But the question now is, which page?