A .obj file is optionally divided into different groups/objects. Each group/object is optionally made up of different meshes, each with it's own material.

So the keys of this dictionary are:

Dict GroupNameOrObjectName (Dict MaterialName Mesh)

If no name is specified in the input file, "default" will be used instead.

A Mesh loaded by the obj loader is a record with a list of vertices and a list of indices. Depending on the mesh type and the loading options you get a different kind of mesh. They differ on what information a vertex contains.

These meshes are meant to be used with WebGL.indexedTriangles mesh.vertices mesh.indices.

Defines a mesh: vertices of any type, and a list of their integer indices

A 3D position with a normal

A 3D position with a normal and a 2D texture position.

The tangent is a vector pointing tangential to the object surface, in the direction of the u texture coordinate. This is needed for doing tangent space normal mapping. The 4th component is either 1 or -1 and has to be used to get the bitangent in the glsl shader, e.g: vec3 bitangent = cross(normal, tangent.xyz) * tangent.w

more info here: https://web.archive.org/web/20160409104130/http://www.terathon.com/code/tangent.html