A Joomla module is an extension that offers certain functionality to a site, that is not supposed to serve as main content, and is usually placed around component code.
The Joomla module tag, would be useful for questions related to Joomla module development. As mentioned by the official Joomla Module documentation, the easiest extension to create is probably a module, which can be as simple as printing the current date and time.
Despite being small and websites can work without modules, Joomla has allowed very important functionalities to be handled by modules, like Menus (mod_mainmenu), Search (mod_search), and even Custom HTML modules (mod_custom) that allow direct HTML input for the unexperienced to write their own HTML in their sites.
To include a module inside an article, Joomla offers a default plugin to do this, but you have to include a full module position. Example code is: {loadposition: module_pos} where module_pos is the module position that you want to include.
Please note that you can add custom module positions, just by writing the desired position name in the position field of the module, instead of selecting from the existing list.
For more information on how to create Joomla modules, please read here.
