After that line either the thread system is initialized, or the program will abort if no thread system is available in GLib, i.e. either G_THREADS_ENABLED is not defined or G_THREADS_IMPL_NONE is defined.
If no thread system is available and vtable is 0 or if not all elements of vtable are non-0, then thread_init() will abort.
To use thread_init() in your program, you have to link with the libraries that the command pkg-config --libs gthread-2.0 outputs. This is not the case for all the other thread related functions of glibmm. Those can be used without having to link with the thread libraries. (You have to link with gthread-2.0 if you actually want to use threads in your application, though.)
A function table of type GThreadFunctions, that provides the entry points to the thread system to be used.