what is structure padding

1#include<stdio.h>
2Struct dummy {
3Int num;
4Double x;
5Float f;
6};
7Struct dummy1 {
8Double x;
9Int num;
10Float f;
11};
12Int main()
13{
14Printf(“size:%d %d
15”, sizeof(struct dummy ), sizeof(struct dummy1));
16System(“PAUSE”);
17Return 0;
18}