An ICP (installation control point) number is an individual identifier of each connection to the electricity network. Each property, whether it be a home or business has an individual ICP.
You can find the ICP number for your property on your previous bill. Or, if you've just moved into a property you should be able to find it on your electricity meter.
We often ask for an ICP number as well as an address as it's the most accurate way of making sure that we'll be billing you for the correct property.