In short, it's a measurement of how much glucose is stuck to proteins in your blood. Hb is haemoglobin (blood) and A1c is the protein.
42 is very good by the way, providing you're not having regular hypos - you are doing very, very well
http://www.diabetes.co.uk/what-is-hba1c.html
Out of interest, when were you diagnosed and what was your initial HbA1c?