A hidden region is a new feature in drupal 7. I was modifying a drupal 7 theme. In the .info file I found two regions - 'page_top' and 'page_bottom'. I wanted to assign some blocks to those regions. But surprisingly I couldn’t find them in block administration area. After some searching I came to know about the feature – hidden region. It's a way to define regions that are available for modules but are not available in the blocks administration area.
By default 'page_top' and 'page_bottom' are hidden regions. More hidden regions can be added via the regions_hidden .info file array.