Simply put, this is the point in time where the majority of heat energy from the burning charcoal/gas or other heat source, is going into the breaking down of fats and connective tissue in the meat. When this happens the internal temperature as read on the remote thermometer will stall for a period of up to several hours depending on the size of the meat being cooked. When the majority of the fats and tissues have been rendered, the meat will resume its climb in internal temperature, and when desired final temp is reached, it can be removed from the smoker.
This is how my father-in-law explained it to me the first time I turned a brisket into a large lump of leather. If anyone else can add to this explanation, by all means, please do!