specifically:
void *Qshow(qytpe);
This void pointer is the same void pointer provided as the 2nd argument to Qadd:
int Qadd(qtype, void *);
The type of data stored in the queue is a void pointer - which is a pointer whose type is unknown. The queue code doesn't know what the pointer points to. It only knows its a pointer.
So here is an example of using the queue:
qtype myqueue;
int i, *p;
myqueue = Qnew(); // create a new queue
i = 4;
Qadd(myqueue, &i); // put a pointer to i in the queue
p = Qshow(myqueue); // get a pointer out of the queue
printf("%d\n",*p); // this should print 4
Dr. Ligon