Modules can add to the set of available features on your site, or change the way existing features work. You can use modules contributed by others or create your own.
Placing downloaded and custom modules in this directory separates them from Backdrop's core modules. This allows Backdrop core to be updated without overwriting these files.
Contributed modules from the Backdrop community may be downloaded at http://backdropcms.org/modules.
You may create subdirectories within this directory to organize your added modules. Some common subdirectories include "contrib" for contributed modules, and "custom" for custom modules. Note that if you change the location of a module after it has been enabled, you may need to clear the Backdrop cache so it can be found. (Alternatively, you can disable the module before moving it and then re-enable it after the move.)
In multisite configuration, modules found in this directory are available to all sites. To restrict modules to a specific site instance, place modules in a directory following the pattern sites/your_site_name/modules.
Refer to the "Developing Modules" section of the developer documentation on the Backdrop API website for further information on extending Backdrop with custom modules: https://api.backdropcms.org/developing-modules.