A JavaScript Engine is a program that executes JavaScript code, allowing web pages to be interactive and dynamic. It takes the code written by developers and converts it into machine language that the computer can understand. Popular engines include Google's V8, used in Chrome, and Mozilla's SpiderMonkey. These engines optimize the code for better performance, enabling faster execution. They also handle tasks like memory management and garbage collection, ensuring that unused data is cleared away. By using a JavaScript engine, developers can create rich web applications that enhance user experience and functionality on the internet.