Much like in TiddlyWiki, TiddlyWeb can use plugins to extend the functionality of the system. Plugins can add functionality to the web server, to twanager or both.
A plugin can be distributed for use with other TiddlyWeb installations. For a list of of plugins, see Plugin List.
There is Python package called tiddlywebplugins.utils that includes useful functions for creating plugins.
A tiddlywebplugins namespace has been reserved for those plugins which have reached a level of stability where it makes sense for them to be distributed from PyPI.
The best way to learn how to create plugins is to look at existing examples.