Smoothing groups are groups of mesh faces which have no sharp edges in between. The normals are averaged over all adjacent faces such that the edge will instead be, well, smoothed. This helps for proper gouraud shading in case more than one face should be seamlessly shaded.
It's usually the question of support in the file format and in the loader if smoothed normals are properly loaded in Irrlicht. Although I wrote some things for the obj loader I don't recall if the meshes support smoothed normals, but .x does and .3ds does also, but the Irrlicht loader discards this information (because it's stored pretty weird and I did not have fun figuring out how to load it).