Power - connect 12 Volts to power the accessory/load and the internal LED
ACC - connect the Load or Relay to ACC (in this case the fog lamp relay)
Ground - is for the internal LED only

The internal LED in that switch uses the 'ground' terminal IE when you throw the switch the LED sees ground and illuminates.
If you don't connect ground the LED won't work but the switch will still operate the relay or load