What is "plugin"
The plugins (or plug-ins), as the name says, serve as "plug-ins", i.e., work to add features to the core software. For example in Photoshop: thousands of plugins available help make it easy to create a complicated effect. In this specific case the plugin can decrease repetitive tasks and speed up creation.
In the navigators, the plugins help to display various types of content, for example: video files, PDF, Java, or even animations, among others.