Gulp.js is a popular open-source toolkit used in front-end web development to automate time-consuming tasks such as file compression, image optimization, and code compilation. It is built on Node.js and uses JavaScript code to define and manage the build processes. Gulp.js has a simple and easy-to-use API that enables developers to create custom build processes by defining tasks and dependencies in a Gulpfile.js configuration file. The tool's main advantage is its ability to streamline the development process by automating repetitive tasks, reducing the risk of errors and increasing productivity. Gulp.js is widely used by developers due to its large ecosystem of plugins and ease of use, making it a valuable asset in modern web development workflows.