Append and insertion of dynamically allocated array

I'm having trouble with this one part of my program dealing with dynamic arrays in C. My teacher provided the prototype functions. I am stuck on the appending part. I just don't understand why p is a double pointer. At first I thought maybe we are suppose to create array of dynamic arrays but then the deletion function does not need a double pointer. This is what I have so far:

#include <stdio.h>
#include <ctype.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>

/*
Name: Marcus Lorenzana
Assignment: HW 4
*/

struct _dynamic
{

    int size;
    int max_size;
    int *data;

};
typedef struct _dynamic dlist_t;

dlist_t * make_dlist(int size);
void app_dlist(dlist_t **p, int value);
int ins_dlist(dlist_t **p, int val, int index);
int del_dlist(dlist_t *p, int index);

int main(int argc, char * argv[])
{

    dlist_t *plist;
    plist = make_dlist(50);



    return 0;
}

dlist_t *make_dlist(int size)
{

    dlist_t *list;
    list = (dlist_t *) malloc (sizeof(dlist_t)); 

    list->size = 0;
    list->max_size = size;

    list->data = (int *) malloc((sizeof(int))*size);


    return list;

}

void app_dlist(dlist_t **p, int value)
{
    /*
    int size;
    size = p->size;

    p->data->size = value;

    size++;
    p->size = size; 
    */



}

int ins_dlist(dlist_t **p, int val, int index)
{

}

int del_dlist(dlist_t *p, int index)
{

}