Cocos2d-x is an open-source game engine optimized for 2D graphics, widely used by over 400,000 developers, and supports multiple platforms including Windows, iOS, and Android. The engine, which was created from a branch of cocos2d in 2010, is notable for its speed and versatility, using C++ for development. The document covers installation, key features, and basic game-building procedures with Cocos2d-x, emphasizing that it has always been free to use.