A game engine is software, or a system of software, for developing a video game.
Game developers often use 3D models as part of the visuals for a game, either as part of a pre-rendered sequence or as interactive elements within the gameplay itself. For example, a player might be able to navigate a scene in real time, moving around a 3D environment that includes several 3D models.
In order to use a 3D model with a game engine, the model must often be set up specifically for use with the game engine. Many game engines will accept models that are only made of quads and triangles (no ngons) and which are textured with a single square texture map for all UVs.