Confusion between ConstPtr vs Ptr

edit

I have recently switched from Python to C++. I was going through the ROS Subscriber tutorials. I came across this piece of code -

void chatterCallback(const std_msgs::String::ConstPtr& msg)
{
  ROS_INFO("I heard: [%s]", msg->data.c_str());
}

I wanted some clarity on ConstPtr.

As per my understanding, we are passing a reference to the msg variable in the chatterCallback function, where in, the msg variable itself is a pointer to a std_msgs::String object.

Assuming my (aforementioned) understanding is correct, why are we using the keyword ConstPtr?

Is it because the value of the msg variable isn't changing (i.e. it is always pointing to the same std_msgs::String object) or it is because the value of the std_msgs::String object (to which the msg variable is pointing to) isn't changing?

I went through this discussion on the forum. But, I am still confused.