JavaScript plugins are reusable pieces of code that extend the functionality of web applications. They allow developers to add features like image sliders, form validation, or interactive maps without having to write all the code from scratch. By using plugins, developers can save time and ensure consistency across their projects. These plugins are often built on popular libraries like jQuery or React, making them easy to integrate. Many plugins are available for free or as open-source, allowing a wide range of developers to enhance their websites with minimal effort.