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.
This panel displays the PID settings for the heater and the bed and also for different temperatures.
To adjust the PID settings manually, tap the minus or plus buttons.
To save them, tap the Checkmark button. The settings are saved in the EEPROM memory of the motherboard.
The cancel any changes, tap the Back button. It goes back to the previous panel.