Plugins are small applications, which are embedding themselves into other applications. By using them you can expand in most cases the set of features your application normally provides.
Since you have landed on this page, you are most likely interested in using plugins for Cura. There is already a set of different plugins, which add a better interaction with CAD software, but also other plugins, which can do other things.
In this section of my website, I would like to introduce you all plugins I created during the time. Some of them are experimental, like the FlashAirPlugin, or get like the CAD integration plugins support as much as I can. Just keep in mind that developing these plugins, distributing and maintaining them for free.
Among with the pages about each plugin, you can also find instructions how to install and remove plugins.