Three.js is a popular JavaScript library that simplifies the creation of 3D graphics in web browsers. It allows developers to build interactive 3D environments using HTML and WebGL, making it easier to render complex scenes without deep knowledge of graphics programming. With Three.js, users can create various 3D objects, animations, and visual effects. The library supports features like lighting, shadows, and textures, enabling the development of immersive experiences for games, simulations, and visualizations. Its extensive documentation and community support make it accessible for both beginners and experienced developers.