Hi and welcome. HbA1c is a measure of the number of glycated red blood cells. This is what happens when a cell has had a glucose molecule attached.
It therfore gives you an estimate of your blood glucose levels over the last three months - because three months is roughly how long a red blood cell lives. The reading is skewed towards recent weeks because some of the oldest cells will have already died.
The Hba1c is a blood test that shows how much glucose has attached to the hemoglobin over the life of the blood cells.
Generally blood cells hang around for about 12 weeks so it's often described as a 12 average..
I believe in other parts of the world it is also called the A1c test
Before the advent of CGMs, it gave a number that represented the "average" (or thereabouts) blood glucose, so was a way of judging whether control was adequate or not. It is still used, lots of recommendations are based on it, however time in range (TIR) is the "new thing" that is possible to see with CGMs. I assume we will also see more recommendations based on metrics derived from CGM data