This function is called by an extension (one of the things hooked into Tcl by load) as the first step of initializing the extension. It's just like a Tcl_PkgRequire of the Tcl library itself (you were doing that already, yes?) but also makes available the rest of the Tcl library functions within the extension. You then compile the extension library with -DUSE_TCL_STUBS set and link against libtclstub.a (which probably has a version number too, and might have a different suffix, e.g., .lib on Windows).
The arguments are: