There is 12Volt Ucc power supply of level indicator. The R3 drives the LED current from IC. The R2 resistor set the sensitivity of indicator and the last resistor R1 set the voltage offset of the first LED. The diode with both R4 and C1 convert the input signal to the DC for IC of level indicator.
Czech resistor values specification:
6k8 = 6800 ΩDatasheets
Display drivers datasheets