Ok, this tag has been used for questions related to developing custom modules, but also for question about the Joomla Custom html module.
Custom Module is a somewhat vague tag. It may refer to the development of a custom module, or to the Joomla's core custom html - a module that allows the user to present information through writing html.
So far, the tag has been used for both types of questions, and generally it seems to be an okay fit.
Developing modules in Joomla is relative easy. For Module Development users may also consult: How do you make a new module?.