A question about fill_free_list(void) function

[Pedram]

Hello community,
I'm reading the CPython interpreter source code,
first, if you have something that I should know for better reading
this source code, I would much appreciate that :)
second, in intobject.c file, I read the following code in
fill_free_list function that I couldn't understand:
<code>while (--q > p)
    Py_TYPE(q) = (struct _typeobject *)(q-1);
Py_TYPE(q) = NULL;
</code>
What does it mean?

Thanks.