• Three.js is a coding tool (specifically, a JavaScript library) that web developers use.
• Its purpose is to make it easier to create and show 3D graphics (like shapes, animations, games) inside a web browser, using code.
• It's like a powerful engine that handles the complicated parts of drawing 3D on the web (using technology called WebGL).
• Unlike Spline's visual editor, with Three.js you build everything by writing JavaScript code. Many complex 3D websites use it "under the hood".