Model files that are placed in the Assets folder in your Unity project are automatically imported and stored as Unity assets.
A model file may contain a 3D model, such as a character, a building, or a piece of furniture. The model is imported as multiple assets. In the Project view the main imported object is a Model Prefab. Usually there are also up to several Mesh objects that are referenced by the Model Prefab.
A model file may also contain animation data which can be used to animate this model or other models. The animation data is imported as one or more Animation Clips.