Hi Big_Daddy,
HbA1c measures your glycosylated hemoglobin. Basically, when our blood glucose levels are high, some of the glucose sticks to the some of the hemoglobin in your blood. The amount depends on how high your BG is. Once it has stuck, it is there until the blood cell is replaced - about 120 days in most people. That means that your HbA1c is a good indicator of average BG levels over the last 3 months, but it is weighted towards the last few weeks.