In Capitan a Module is a set of functions and tools that can be added to your platform to enable you to create forms, galleries and common "repeat format" content.
Things like news and blog feeds, events and vacancy listings, knowledge bases, and online stores. The activated module provides an intuitive interface enabling you to create categories and populate them with articles/events/vacancies/products.
The beauty of creating repeat format content through a module is that Capitan will automatically create listings pages with pagination controls, and you have full control over the design of both the listing pages and the articles/events/vacancies/products.
In other platforms this kind of functionality requires the installation of "plug-ins" which are pieces of code that have been developed by third-parties, not the orginal platform developers. Using plug-ins in other platforms can pose security risks and lead to problems when you update your system.
Capitan modules are developed by Capitan, so these kind of issues can be avoided.
Capitan has a selection of free modules which can be installed via work request, and premium modules which you can purchese in the online store.