According to Wikipedia, a plug-in (or plugin, add-in, addin, add-on, or addon) is a software component that adds a specific feature to an existing computer program. When a program supports plug-ins, it enables customization.
Quite often, Open Source software supports plug-ins. WordPress (WP), the leading Content Management system (web site development tool) supports thousands or plug-ins, also called Widgets in WP lingo.
Using plug-ins and addons extend the usage of some Open Source applications exponentially.
Noteworthy: Developers of Open Source software offer free plug-ins, as Open Source, and in addition, sell certain plug-ins, as a source of income. Look up: Freemium