The Writer theme, like all Drupal themes, contains several block regions. These regions represent places in the layout of your site that you can drop blocks of content into. Here is a map of which regions are available to you:
A simple blog probably won’t need most of these regions. This example site only uses two… Content (which contains the words you are currently reading) and Footer Message. Regions not in use will collapse down and become invisible, so it doesn’t hurt to have plenty of them in case you ever need them.
For more information on using block regions, see: http://drupal.org/documentation/modules/block.