A plugin is a software add-on that is installed on a program, enhancing its capabilities. For example, if you wanted to watch a video on a website, you may need a plugin to play it because your browser doesn't have the tools it needs. You can think of it like getting a Blu-ray player for your Blu-ray disc.
Examples of plugins
What's the difference between a plugin and an extension
While both plugins and extensions add functionality to browsers there are a few differences. Plugins only know about the page in which they're embedded; they work the same across all browsers, but are specifically inserted into a page. Extensions, on the other had, can gather information from a browser and can be used across all pages.