C++ help

Could someone help me out with "private" in object programming? I just don't get the use of it. I understand declaring things in private is supposed to make it "private", but I just can't think of a good way to use it or how it's actually "private". This is one of the examples my teacher gave me. He doesn't speak english very well which is why I had to make this thread and ask for SP's help.

Code:

#include<iostream>
using namespace std;
// declaration section:
class Student
{
    private:
       int idNum;
    public:
       static double athleticFee;
       void setIdNum(int);
       void displayStudentData();
};
// implementation section:
double Student::athleticFee = 45.25;
void Student::displayStudentData()
{
  cout<<"Student #"<<idNum<<"'s athletic fee is $"
    <<athleticFee<<endl;
}
void Student::setIdNum(int num)
{
   idNum = num;
}
int main()
{
     Student aFreshman, aSophomore;
     aFreshman.setIdNum(1234);
     aSophomore.setIdNum(2345);
     aFreshman.displayStudentData();
     aSophomore.displayStudentData();
  cout<<"The standard student athletic fee is $"<<
   Student::athleticFee<<endl;
}

Now what I don't understand is how idNum (declared in the object/class) is private. When I run the program, it straight up tells me what the idnum is of aFreshman and aSophomore, so how is that private?