Zikula uses a modular design that allows you to add to the functionality by installing "modules".
Just like you would install an application on your home computer like a work processor, FTP client, music player, or game to add to the functionality and allow you to perform a task, modules perform a task within the Zikula framework.
Some modules that you could install (if they're not already installed) would be page creators, forums, image management, etc.