As you have seen in the "Introduce to layout, element in a Symphony site" page, there is a home page layout, which has many blocks like Recent news, Polls, User login, Who's new etc ... Blocks are the boxes of content that can be displayed in regions (such as footer or sidebar) on your page.

Blocks are made available to your site most commonly by enabling modules. Once created, a Block can be modified to adjust its appearance, shape, size and position - or which pages it appears on. For example, enabling the core Poll module makes the "Most Recent Polls" block available for you to place in a region. Also note that some modules provide multiple blocks when enabled, others may not define new blocks.

Go to Admin > Structure > Block (admin/build/block).

Master Drupal in 7 hours - Basic Customization - Manage block

This section shows you how to create/edit/delete a block with your own content and change its position.