Minicomponents are designed to be discovered and used by Jomres to add, provide, or override functionality in the system. In simpler terms, they're discreet sections of software that perform a certain function. They are similar to hooks in Wordpress and Events in Joomla such as onContentPrepare. I originally came up with the idea of using trigger numbers back in the summer of 2005. Since then several times I have considered replacing the numbers with more descriptive event names but the number system is well understood, well documented so it's not really necessary to refactor them.
The most complete, uptodate list of these trigger numbers can be found in the /core-minicomponents directory on Github.
Jomres comes with a core set of minicomponents, which are installed in the /jomres/core-minicomponents directory. If you are looking to modify any core minicomponent, whilst the license (GPL/MIT) of the system allows you to do so, we would discourage it. Instead, you should copy any file you want to edit to the /remote_plugins directory and make changes to the file there. More information is available in the Customising Jomres section.
The term Minicomponents is derived from Jomres' history. In the past, when we devised how to add functionality on the fly we would install this additional functionality via the Joomla component installer, hence the term minicomponents.
Nowadays a minicomponent is a file who's naming convention follows the style jNNNNNXXXXX.class.php (for example j00006sanity_checks.class.php), and is usually installed via the Jomres Plugin Manager, but it's also possible for you to create the file "in situ" and use the Rebuild Registry feature to tell Jomres that it exists.