What is plug-in?
A plugin (plug-in, addin, add-in, addon or add-on) sometimes also called extension, is a computer program that interacts with a host application (a web browser or an email client, for example) to provide a certain, usually very specific, function “on demand”.
Applications support plugins for many reasons. Some of the main reasons include: enabling third-party developers to create capabilities to extend an application, to support features yet unforeseen, reducing the size of an application, and separating source code from an application because of incompatible software licenses.
In other words, A plug-in is an application built into a browser or added to a browser to enable it to interact with a special file type (such as a movie, sound file, Word document, etc.)