Areas of your web site are defined in "_areas" folder. Areas are usually used in templates to specify where common web site elements are located. Areas are PHP files with common pieces of your web site. They have "are" extension. Areas often refenence modules. For example if you have a module "topmenu" and want to put into "top" area of your web site, you can create "top" area by placing the following content into "_areas\top.are":
[MODULE]topmenu[/MODULE]
You do not necessary need to use modules inside areas. You can place any HTML/PHP code there. For example you may want just to have a few hyperlinks there:
[LINK art=about]About My Site[/LINK]
[LINK art=help]Help[/LINK]
Areas are important elements of a TextCMS web site, but they are implemented as an internal plugin. Please check out the AREA plugin documentation.