Question regarding pthread creation in linux

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Linux

C++98

I am learning c++ and came across below code in which I did not understand below line.
rc = pthread_create(&threads[i], NULL, PrintHello, (void *)&td[i]);

Where,
rc = return code in int
thread is pthread_t threads[5]
PrintHello is threadRoutine
td is structure with 2 variable 1 is int thread_Id and other is char* msg

I did not understand why (1st argument) is passed as a reference and (last argument) td typecasted to (void*)

Complete code:
struct thread_data {
int thread_id;
char *message;
};

void *PrintHello(void *threadarg) {
struct thread_data *my_data;
my_data = (struct thread_data *) threadarg;

cout << "Thread ID : " << my_data->thread_id ;
cout << " Message : " << my_data->message << endl;

pthread_exit(NULL);
}

int main () {
pthread_t threads[NUM_THREADS];
struct thread_data td[NUM_THREADS];
int rc;
int i;

for( i = 0; i < NUM_THREADS; i++ ) {
cout <<"main() : creating thread, " << i << endl;
td[i].thread_id = i;
td[i].message = "This is message";
rc = pthread_create(&threads[i], NULL, PrintHello, (void *)&td[i]);

if (rc) {
cout << "Error:unable to create thread," << rc << endl;
exit(-1);
}
}
pthread_exit(NULL);
}

What I have tried:

Read references but did not understand why it is necessary here