Hi Tracie,
*Thieving* is copper added to outer layers of a PCB to create a uniform distribution of copper across the surface. The reason this is done is to make sure the copper plating in the holes is uniform. If the copper distribution in the artwork is not uniform, areas with little exposed copper will plate very heavy while areas with large amounts of copper, such as BGAs or connector pin fields will not plate properly. So, “thieving” steals some of the plating current that would otherwise concentrate on features that are sparse and be spread thin in areas with areas that are dense with features.
You could check the whole article Here
Hope it helps
