This simply creates a deep copy of the original mesh and saves memory by sharing the geometry. Each clone can have its own material and transformation.
Note that as of v5.0, cloning a mesh with skeleton will assign said skeleton to the clone, ie, the skeleton is shared. If your clone does not need a skeleton, simply set it to null.
clone.skeleton = null;
Alternatively, if your clone needs a skeleton, simply clone's the source mesh's skeleton.
clone.skeleton = mesh.skeleton.clone();