I was talking with Jake yesterday about smoking meats and we got to the subject of the brisket we did on Thanksgiving and the length of time it took, etc. I told him you can expect a brisket to cook at a rate of about 1.5 hours per pound at an average of 230 degrees. He did the math and then asked why it took an extra 40 minutes or so to reach our temp. I told him it was the dreaded plateau and that it was not to be feared. The next question was inevitable. That got me to thinking of all the new folks here and that we should probably breach the subject again just so no-one panics and throws coal to the fire.
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!