Here is a simple (I hope) explanation. Think of a large stand of woods bordered on one side by a road. The other side of the road you have a strip of woods with fields on both sides. The strip of woods would be considered a funnel. Deer will use this strip to stay conceled. Therefore you have your funnel. It could be a creek bottom, a draw between to ridge lines, or a strip of woods between a couple of fields. Hope this helped.