The HbA1C is your average blood glucose levels in your blood over the last 3 to 4 months. Doctors use it in regular checkups to make sure something weird isn't happening to you. Generally speaking, you should be between 4.44 and 6.66. The "average" range of healthy people is between 4 and 8.
Diabetics generally have bad HbA1C and over the decades doctors have harassed diabetic patients when their HbA1Cs aren't improving or are slipping. This kind of thinking is starting to disappear thankfully, but HbA1C is really very important. A HbA1C of anything over 10 guarantees you'll have medical complications later in life - and by later, I mean as soon as 20-30 years from now.
Programs like DAFNE (Dose Adjustment for Normal Eating) try to get diabetic HbA1Cs to a target of 7.5 - which is pretty difficult for many Diabetics who have been taught how to manage their diabetes differently for decades. Instead of changing your meal to match a certain number of carbs, you change your insulin dosage to match the number of carbs in a meal. I've not learnt DAFNE, but I intend to go on the course to find out -what I don't know-. Why am I sounding so confident? For this reason:
HbA1C when diagnosed with Type 1 Diabetes: > 15% (very very bad)
HbA1C last time I saw the doctor: 10% (doctor was very pleased)
HbA1C today: 7.2% (doctor is ecstatic!)