Mugen is a free, open-source 2D fighting game engine that allows users to create and customize their own fighting games. It supports a wide range of characters, stages, and game mechanics, enabling players to design unique gameplay experiences. The engine is popular among fans of fighting games due to its flexibility and extensive community support. Users can create characters by coding their moves and attributes, often using sprites from various sources, including other games and original designs. The Mugen community shares content online, allowing players to download and add new characters and stages, enhancing the overall gaming experience.