An earth ground. The ground of your household plugs is supposed to go to an earth ground.
Because the 3 hole grounded plugs of today were not always the "standard", some folks "cheated" to use the 3 prong outlets with only two wires, tying that ground hole to the nuetral. (not right, but better than nothing at all) l. Sometimes done to avoid adptors, or because they wanted to fool inspectors (rather than run new wire).. etc.,. Anyhow, you can't be 100% shure without checking. Some cheap "plug check tools", don't work well. Make sure you buy quality or have an electrician check to see if you are truley earth grounded at all outlets.
