Int I=5, *p; p=&I; scanf("%d", p); printf("%d %d\n", I, *p);

p is a pointer to I and scanf needs a pointer n order to know where to put the value it reads.
This is normal, because in C everything is passed t a function by value not by reference - which means that the function gets a copy of the variable rather than the variable itself. That makes a lot of sense if you thin about it:

void foo(int x)
   {
   x = x +1;
   printf("%d\n");
   }

Is fine, but what would happen if you called it like this:

foo(666);

If you didn't take a copy of the value, you would have the code trying to change the value of a constant number!

So if you want to change something in the calling function from the called function, you have to pass a pointer rather than the value, so a copy of the pointer is passes which points at the same place as the original.