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 hand, can gather information from a browser and can be used across all pages.
Should I use "plugin" or "plug-in" in my writing?
Today, both versions of these words are correct. Technically speaking "plug-in" (with the hyphen) is the correct way to write this word and it's also in the Microsoft Manual of Style as "plug-in." However, "plugin" is the most widely used version of this word and most people and companies have opted for the non-hyphenated version of this word.