I am trying to reverse the stack using recursion. What is being passed in function fun()? s is an object of class stack

// Reverse the stack with fun() and fun1()  
/* I am unable to understand what is being passed in function fun(). 
   I am not sure about the argument of type stack <int> &s. 
   Is it a whole stack being passed or just one node? */     

void fun1(stack<int> &s, int k)
{
   if (s.empty())
   {
       s.push(k);
       return;
   }

    int t = s.top(); s.pop();
    fun1(s, k);
    s.push(t);
}

void fun(stack<int> &s)
{
    if (s.empty())
    {
        return;
    }

    int t = s.top(); s.pop();

    fun(s);
    fun1(s,t);
}

The program is trying to reverse the stack using the functions fun() and fun1(). My question is only that the argument stack <int> &s takes the address of what?

In C++ a reference is basically an alias for an existing variable. They are represented using the ampersand (&); stack<int>& indicates a reference to a stack<int>. Thus the arguments named s in your functions fun and fun1 indicate references to entire stacks, not individual nodes thereof.