What exactly does the parameters p_x,p_y and p_size obtained as a result of the camera calibration mean ?

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I have used the ROS camera_calibration package provided in the ROS website. The package is working properly but I'm not able to understand what the does parameters p_x,p_y & p_size implies in the output obtained from the packages. Also in the source code of the camera calibration package written in python, I didn't understand how the above parameters are calculated by the author ?

The output obtained is according to the image attached.

Links:- http://wiki.ros.org/camera_calibratio...

Any help in these problem will be highly appreciated and thanks in advance.![image description]