Touch screen calibration explanation borrowed from three sites:
Touch-screen input devices are actually entirely separate devices from the display screens that they overlay. As a result, there is no built-in relationship between the coordinates of a spot on the display screen and the coordinates sensed when someone touches directly over that spot. Instead, the software for the touch-screen interface must learn which spots on the touch sensor overlay which spots on the screen. This is called touch-screen calibration.
If there is any need to calibrate the touch screen monitor, it must be done with the manufacturer documentation or support services.
To calibrate the machine normally, you typically go through a ritual where you are asked to touch a target at at least three locations on the screen, frequently two opposite corners and one central spot.