Check the NEC (National Electric Code) or your local electrical code. It is a secondary ground for the structure's electrical system. Around here they run a rebar or copper wire, in the footing pour, approximately 2" above grade. The bar length is around twenty feet and needs to be in place prior to the footing pour. The structure's electrical system is then grounded to the bar (or copper wire).
When I was the general contractor, we used the rebar option and I had the concrete subcontractor install the rebar. Custom probably varies in different locations.
