The meaning of the " istringstream is" statement ?

An open source code, use the following loop to the user input:

while (token != "quit")
  {
    if(!getinline(cin,string X))
           string x="quit";
    //code for user input..
    istringstream is(string X);
    //go code in run ..
  }

What does the "istringstream is"  statement?

beginner of planning