From RepRap web site:
PID tuning refers to the parameters' adjustment of a proportional-integral-derivative control algorithm used in most repraps for hot ends and heated beds.
Select the target temperature you want to use (with the plus and minus buttons)
Tap the Continue button to start the PID Tuning process.- The printer increases and decreases the temperature of the hot-end around the target value.
- When the PID tuning is finished (after some minutes), the new PID values are displayed:
Tap the Back button the restart the process.
Tap the Checkmark button to save the settings. They are saved in the EEPROM memory of the motherboard.
Note: Contrary to previous versions of ADVi3++, you can only perform the PID tuning god the extruder, not if the bed. PID tuning of the bed is not so important and the removale of this feature gives more memory (SRAM) for other features. For the same reason, there is only one set of PID values recorded.