This is a good description of the test I’m due tomorrow.  From the article,

In the blood stream are the red blood cells, which are made of a molecule, haemoglobin. Glucose sticks to the haemoglobin to make a ‘glycosylated haemoglobin’ molecule, called haemoglobin A1C or HbA1C. The more glucose in the blood, the more haemoglobin A1C or HbA1C will be present in the blood.

Red cells live for 8 -12 weeks before they are replaced. By measuring the HbA1C it can tell you how high your blood glucose has been on average over the last 8-12 weeks. A normal non-diabetic HbA1C is 3.5-5.5%. In diabetes about 6.5% is good.