can't understand the header <sstream>

[nasser]

i have read it a lot on this site but can not understand the header <sstream>
I know many of basics like arrays ,functions etc etc...
but can not understand why we use this header
Code from this site

 // stringstreams
#include <iostream>
#include <string>
#include <sstream>
using namespace std;

int main ()
{
  string mystr;
  float price=0;
  int quantity=0;

  cout << "Enter price: ";
  getline (cin,mystr);
  stringstream(mystr) >> price;
  cout << "Enter quantity: ";
  getline (cin,mystr);
  stringstream(mystr) >> quantity;
  cout << "Total price: " << price*quantity << endl;
  return 0;
}

[coder777]

This header is required for stringstream:

http://www.cplusplus.com/reference/sstream/stringstream/?kw=stringstream