In the UK, and probably elswhere it means that the cable is suitable for up to 600 volts from any conductor to ground/earth and also for up 1000 volts between any conductors.
The voltages may be assumed to be AC RMS unless stated otherwise.
It would commonly be used on UK 240/415 volt 3 phase 4 wire systems, but could of course be used on other voltages subject to the limits above.
