Box2D is a popular open-source physics engine used for simulating 2D rigid body physics in games and applications. It provides developers with tools to create realistic movements and interactions between objects, such as collisions, gravity, and friction. Designed for performance and flexibility, Box2D is widely used in game development, particularly in platforms like Unity and Cocos2d. Its robust features allow for the creation of complex physical environments, making it easier for developers to implement engaging gameplay mechanics.