SPINE 3D is a 2D animation tool used for creating skeletal animations, typically for games. It focuses on creating fluid, realistic character animations through a bone-and-skeleton system. SPINE allows developers to create complex animations by manipulating parts of characters independently, offering a high level of flexibility and control over the movement. It is compatible with various game engines like Unity and Unreal, making it an essential tool for 2D game developers. SPINE also supports exporting animations in formats that can be easily integrated into game engines, making it a popular choice for animating 2D characters in video games.