Addon modules allows developers to extend the functionality of WHMCS further including new pages in the admin or client areas. Addons can be obtained from the in-built Addon Directory by clicking the Addons tab or via http://whmcs.com/appstore
Once downloaded and installed they are all managed through the Setup > Addon Modules interface. Once activated, the modules will display in the dedicated "Addons" dropdown menu within the admin area for quick & easy access from any page.
Management options consist of activating and deactivating of the modules, aswell as access control which allows full admins to define exactly which of the administrator roles are allowed to access each addon module.
Documentation for creating an addon module can be found at Addon Modules
Downloading and installing an addon
Addon module management & access control