are the additional pieces you can plug into your pages: modules are built-in applications that you can place anywhere on your pages.
Modules are prebuilt program-software pieces waiting to be called by your commands in any wiki-page and is replaced in the rendered output of the page with the output of the mini-application.
Some modules have the possibility in the output to be changed to your wished layout, some not;
some big modules reads all your saved pages-content , some only the forum…
(See the documentation on http://www.wikidot.com/doc:modules )