refers to object file. Wavefront .obj file, a 3D geometry definition file format with .obj file extension. he file format is open and has been adopted by other 3D graphics application vendors. For the most part it is a universally accepted format.
This file format is a simple data-format that represents 3D geometry alone — namely, the position of each vertex, the UV position of each texture coordinate vertex, vertex normals, and the faces that make each polygon defined as a list of vertices, and texture vertices.
Materials that describe the visual aspects of the polygons are stored in external .mtl files. More than one external MTL material file may be referenced from within the OBJ file. The .mtl file may contain one or more named material definitions.