A WordPress plugin is a piece of software that contains a list of functions which can be added to a WordPress-based website. They can actually extend a functionality or add new features to your WordPress website. Those plugins are usually written in PHP and integrate very well with the website.
WordPress plugins let their users easily modify, customize, and enhance your website. You can also create a plugin yourself. Instead of modifying the core software your WordPress website is built, you can rather build one to extend it. Utilizing plugins lets you have the most optimized WordPress installation in the process of maximizing how you utilize the software to start with.