overloaded operator question

[KvltKitty]

Okay so I have a pretty simple question. I'm just trying to figure out what exactly this one variable is in an example we were given to help us understand overloaded operators. It's called input (under the overloaded operator function). I just am not seeing what exactly it is. Here are the three files we were given to examine (I cut them down to just what you need I believe):

Student.h

#include<string>
#include<vector>
using namespace std;

#ifndef STUDENT_H
#define STUDENT_H

class Student{
	friend ostream &operator<<(ostream &,  Student &);
	friend ifstream &operator>>(ifstream &, Student &);
	
	public: 
		Student(){};
		string get_name();
		void get_my_courses( vector<string> &courses);
		bool operator<( Student & );
	private:
		string last;
		string first;
		vector<string> my_courses;
};

#endif 

student.cpp

ifstream &operator>>(ifstream &input, Student &some){
	string s;
	getline(input, s);
	istringstream istr(s);
	istr >> some.last;
	istr >> some.first;
	vector<string> v;
	while( !istr.eof()){
		istr >> s;
		if(s != "")
			v.push_back(s);
	} 
	some.my_courses = v;
	return input;	
}

ostream &operator<<(ostream &out,  Student &some){
	out << "Student: " << some.get_name() << endl;
	out << "Courses taken: " ;
	for(int i = 0; i < some.my_courses.size(); i++)
		out << some.my_courses[i] << " ";
	out << endl;
}

student_main.cpp

#include<iostream>
#include<fstream>
#include<string>
#include<vector>
using namespace std;

#include "student.h"

int main( int argc, char *argv[]){
	if(argc < 2)
	{
		cout << "Enter input file" << endl;	
		exit(0);
	}
	string input_file = argv[1];
	ifstream inf;
	inf.open(input_file.c_str());
	
	Student ast, ast2;
	inf >> ast;
	cout << "The first student: " << endl << endl;
	cout << ast << endl;
	inf >> ast2;
	cout << "The second student: " << endl << endl;
	cout << ast2 << endl;
	if( ast < ast2 )
		cout << ast << "is alphabetically preceding\n" << ast2 << endl;
	else 
		cout << ast2 << "is alphabetically preceding\n" << ast << endl;
	inf.close();

}