What does it mean to be a private member (c++ classes)?

I am a little confused about private data members in C++ classes. I am new to coding and still in the middle of my 'Classes' chapter so I might be ahead of myself, but I feel like I am missing a piece of information:

Let's say I have this code:

class clocktype;
{ 
  public:
          void setTime(int,int,int);
          .
          .
          .
   private:
          int hr;
          int min;
          int sec;
     };

And I create a object myclock.

clocktype myclock;
myclock::setTime(hour,minute,min)
{
  if (0<= hour && hour < 24)
  hr = hour;

  if (0<= minute && minute <60)
  min = minute;

  if ( 0<= second && second<60)
  sec = second;
 }
 myclock.setTime(5,24,54);

My textbook says I can't do this:

myclock.hr = 5;

because hr is a private data member and object myclock has only access to the public members. But isn't that practically what I am doing in my setTime function anyways? I understand I am accessing myclock.hr through my public member function. But I am still struggling with the logic behind that. What does it mean to be a private data member then?